Fixing Docker Error: "conflict: unable to remove repository reference"

Have you ever tried to clean up your local machine by deleting old Docker images, only to be met with this frustrating message?

Error response from daemon: conflict: unable to remove repository reference

"my-image" (must force) - container <ID> is using its referenced image <ID> 

This error happens because Docker is protective. It won't let you delete an image if there is a container—even a stopped one—that was created from it.

Step 1: Identify the "Zombie" Containers

The error message usually gives you a container ID. You can see all containers (running and stopped) that are blocking your deletion by running:

docker ps -a

Look for any container that is using the image you are trying to delete.

Step 2: Remove the Container First

Before you can delete the image, you must remove the container. If the container is still running, you’ll need to stop it first:

# Stop the container

docker stop <container_id>

# Remove the container 

docker rm <container_id>

Step 3: Delete the Image

Now that the dependency is gone, you can safely remove the image:

docker rmi <image_name_or_id>

The "Shortcut" (Force Delete)

If you don't care about the containers and just want the image gone immediately, you can use the -f (force) flag.

Warning: This will leave "dangling" containers that no longer have a valid image reference.

docker rmi -f <image_id>

Pro Tip: The Bulk Cleanup

If your machine is cluttered with dozens of these conflicts, don't fix them one by one. Use the prune command to safely remove all stopped containers and unused images in one go:

docker system prune

(Add the -a flag if you also want to remove unused images, not just "dangling" ones.)

How to dockerize your python application in docker

Dockerize your python application:

Docker is a technology which lets you build, deploy and run your applications. Docker enables you separate your infrastructure from your application. With Docker all you need to do is just write your code,.

dockerize it and distribute it in form of image. That way any one can use your application who is running the Docker.

What do you mean by Dockerize application?

Dockerize mean you write your code on your system then you prepare the image and distribute it over the internet or on DockerHub. You don't have to worry about the underlying infrastructure and dependencies.

Let's write a python program which will count the occurrence of words from a given string.

#Input : string = "Docker is a technology which

# lets you build, deploy and run your applications.";

#Count occurence of words from a given string example

def findFreq(s):

dictt = {}

strng = s.split(" ")

strr1 = set(strng)

for word in strr1:

dictt[word] = s.count(word)

return dictt

if __name__ == "__main__":

x = input("Enter your string:")

#raw_input in python 2.x and input() in python 3.x

print(findFreq(x))

#Output: {'a': 4, 'and': 1, 'run': 1, '': 80,

# 'deploy': 1, 'technology': 1, 'is': 1,

# 'you': 2, 'lets': 1, 'applications.': 1,

# 'which': 1, 'build,': 1, 'Docker': 1, 'your': 1}

Save this file with findfrequency.py in same directory. I am saving it in current directory for my convenience but you can save it anywhere and pass the absolute path.

Now lets create a Dockerfile.

FROM python:3

We need to use python in docker so we are using FROM keyword so this will create layer from python image. Means your image is based on python image. 

Now we need to run our python file so we need to add this file to Dockerfile.

ADD findfrequency.py /

Use CMD to execute commands when image loads

CMD ["python", "./findfrequency.py"]

Combine all above lines and create a Dockerfile.

FROM python:3

ADD findfrequency.py /

CMD ["python", "./findfrequency.py"]

So we have created a Dockerfile now. I saved it with the name "Dockerfile" in current directory. When you run docker build .     command then docker looks for Dockerfile if ithis file doesn't exist or file name is wrong or extension is wrong you'll get file not exists error.

Now we are ready to build image from the dockerfile. 

Open the terminal and run the below command and make sure you are in the same directory where you saved your Dockerfile as well as python file.

docker build -t myapp .

-t : This is tagging a name to your image. In this case I gave my image a name "myapp"

.(dot) : Is current directory

Ok so you have successfully build your image. Now Let's check what's inside the image by inspecting it.

docker inspect myapp

[

    {

        "Id": "sha256:c4595feabbd0b9aba4ae67037ea3c43a8c0aaf2abe6f6fd28d25b22a7cf9",

        "RepoTags": [

            "myapp:latest"

        ],

        "RepoDigests": [],

        "Parent": "",

        "Comment": "buildkit.dockerfile.v0",

        "Created": "2021-10-01T08:42:53.450488763Z",

        "Container": "",

        "ContainerConfig": {

            "Hostname": "",

            "Domainname": "",

            "User": "",

            "AttachStdin": false,

            "AttachStdout": false,

            "AttachStderr": false,

            "Tty": false,

            "OpenStdin": false,

            "StdinOnce": false,

            "Env": null,

            "Cmd": null,

            "Image": "",

            "Volumes": null,

            "WorkingDir": "",

            "Entrypoint": null,

            "OnBuild": null,

            "Labels": null

        },

        "DockerVersion": "",

        "Author": "",

        "Config": {

            "Hostname": "",

            "Domainname": "",

            "User": "",

            "AttachStdin": false,

            "AttachStdout": false,

            "AttachStderr": false,

            "Tty": false,

            "OpenStdin": false,

            "StdinOnce": false,

            "Env": [

                

                "LANG=C.UTF-8",

                "PYTHON_VERSION=3.9.7",

                "PYTHON_PIP_VERSION=21.2.4",

                "PYTHON_SETUPTOOLS_VERSION=57.5.0",

                "PYTHON_GET_PIP_SHA256=fa6f3fb93cce234cd4e8dd2be9c247653b52855a48dd44e6b21ff28b"

            ],

            "Cmd": [

                "python",

                "./findfrequency.py"

            ],

You'll see output something like above. Our python function is there inside the output under CMD tag.

Let's run the image.

docker run -it myapp   

Enter your string: This is my test to test dockerfile. 

{'': 37, 'is': 2, 'dockerfile.': 1, 'to': 1, 'my': 1, 'test': 2, 'This': 1}

See the output above and pass the desired string to count the words.

So we have successfully dockerized our application. You can send this image to others so that they can use your program and they don't have to worry about installing any dependencies which can cause your program to crash. 

How to create a Dockerfile to automate the build image process

 Creating a Dockerfile

Dockerfile is a text file which contains set of commands which are used by user to build an image via command line. Dockerfile is an automated way to build an image with set of commands.

Docker build is a command which builds an image from Dockerfile. Let's create a docker file and put some commands inside it. 

touch Dockerfile

Above touch command is used to create a new file on mac/linux. Make sure you create the docker file in current directory. Although you can create the docker file anywhere but for convenience I am creating it on current directory. So that I don't have to type the absolute path for docker file.

Always remember these three commands while creating a new docker file - 

FROM : This creates a layer from docker image

RUN : builds your application

MAINTAINER : Best practice to put the name of maintainer of the image

CMD : Run the commands inside container 

I have created the dockerfile and put the below text inside dockerfile.

FROM ubuntu

MAINTAINER DATATIPSS

RUN apt-get update

CMD ["echo","This is my first image using dockerfile"]

I am using ubuntu as base image and giving maintainer as datatipss. You can give your name. 

Now run the docker file using docker build -t myapp .  command

Here -t : tag your image or giving a name to your image and .(dot) is running dockerfile in current directory.

docker build -t myapp .

[+] Building 79.3s (6/6) FINISHED                                                                                                                             

 => [internal] load build definition from Dockerfile                                                                                                     0.0s

 => => transferring dockerfile: 152B                                                                                                                     0.0s

 => [internal] load .dockerignore                                                                                                                        0.0s

 => => transferring context: 2B                                                                                                                          0.0s

 => [internal] load metadata for docker.io/library/ubuntu:latest                                                                                         7.9s

 => [1/2] FROM docker.io/library/ubuntu@sha256:9d6a8699fb5c9c39cf71bd6219f0400981c570894cd8cbea30d3424a31f                                          7.3s

 => => resolve docker.io/library/ubuntu@sha256: 9d6a8699fb5c9c39cf71bd6219f0400981c570894cd8cbea30d3424a31f                                          0.0s

 => => sha256: 9d6a8699fb5c9c39cf71bd6219f0400981c570894cd8cbea30d3424a31f 1.42kB / 1.42kB                                                           0.0s

 => => sha256: 9d6a8699fb5c9c39cf71bd6219f0400981c570894cd8cbea30d3424a31f 529B / 529B                                                               0.0s

 => => sha256:54ab604fab8d1b3d1c8e02509cc7031f8541428051401f4122619e5968e16 1.48kB / 1.48kB                                                           0.0s

 => => sha256:ab2d02b1ec420fdb84c9f52abda403b6a0f5de904a2ecda5ae4f5cd6e4d46 27.17MB / 27.17MB                                                         6.6s

 => => extracting sha256:ab2d02b1ec420fdb84c9f52abda403b6ae0f5de904a2ecda5ae4f5cd6e4d46                                                                0.7s

 => [2/2] RUN apt-get update                                                                                                                            63.9s

 => exporting to image                                                                                                                                   0.0s

 => => exporting layers                                                                                                                                  0.0s

 => => writing image sha256:76ff7254c9ca346bfe34c42c3f7c17ad0958e70210f01f9e200cbdadb2e4f   

Ok now you have successfully created your image. Let's check and run the image using docker run imageid_or_image_name command.

Check images first:

docker images

REPOSITORY        TAG       IMAGE ID       CREATED         SIZE

myapp             latest    76ff7254c9ca   3 minutes ago   92.2MB

Now run the image:

docker run myapp

This is my first image using dockerfile

So we have successfully created our first image and ran it. 

Also you can inspect the image and get insights of it. If you are using image build by someone else so before running it you can always inspect it to check if it is legitimate image. You can inspect any image and check the author and what commands are inside the image.

In this case you can check the author is datatipss and under cmd tag it's echo command only.

docker image inspect myapp

[

    {

        "Id": "sha256:76ff7254c9ca346bfe34c42c3f7c1958e70210f01f9e200cbdadb2e4f",

        "RepoTags": [

            "myapp:latest"

        ],

        "RepoDigests": [],

        "Parent": "",

        "Comment": "buildkit.dockerfile.v0",

        "Created": "2021-09-28T17:40:34.86456821Z",

        "Container": "",

        "ContainerConfig": {

            "Hostname": "",

            "Domainname": "",

            "User": "",

            "AttachStdin": false,

            "AttachStdout": false,

            "AttachStderr": false,

            "Tty": false,

            "OpenStdin": false,

            "StdinOnce": false,

            "Env": null,

            "Cmd": null,

            "Image": "",

            "Volumes": null,

            "WorkingDir": "",

            "Entrypoint": null,

            "OnBuild": null,

            "Labels": null

        },

        "DockerVersion": "",

        "Author": "DATATIPSS",

        "Config": {

            "Hostname": "",

            "Domainname": "",

            "User": "",

            "AttachStdin": false,

            "AttachStdout": false,

            "AttachStderr": false,

            "Tty": false,

            "OpenStdin": false,

            "StdinOnce": false,

            "Env": [

                "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

            ],

            "Cmd": [

                "echo",

                "This is my first image using dockerfile"

            ],

            "ArgsEscaped": true,

            "Image": "",

            "Volumes": null,

            "WorkingDir": "",

            "Entrypoint": null,

            "OnBuild": null,

            "Labels": null

        },

        "Architecture": "arm64",

        "Variant": "v8",

        "Os": "linux",

        "Size": 92173773,

        "VirtualSize": 92173773,

        "GraphDriver": {

            "Data": {

                "LowerDir": "/var/lib/docker/overlay2/76ff7254c9ca346bfe34c42c3f7c1958e70210f01f9e200cbdadb2e4f/diff",

                "MergedDir": "/var/lib/docker/overlay2/c7dp26x65obhv96p76ff9lbo1/merged",

                "UpperDir": "/var/lib/docker/overlay2/c7dp26x65obhv96p76ff9lbo1/diff",

                "WorkDir": "/var/lib/docker/overlay2/c7dp26x65obhv96p76ff9lbo1/work"

            },

            "Name": "overlay2"

        },

        "RootFS": {

            "Type": "layers",

            "Layers": [

                "sha256: 76ff7254c9ca346bfe34c42c3f7c1958e70210f01f9e200cbdadb2e4f",

                "sha256:ecd02c9154cf7dc6fa2257f9378687064ebbb87524aa7191a683650b0c"

            ]

        },

        "Metadata": {

            "LastTagTime": "2021-09-28T17:43:44.89023209Z"

        }

    }

]

That way you can check what's going to run from the image. Once you build your image you can make it publicly available for everyone. You can upload it on DockerHub and anyone can use it. 

You can build multiple images of multiple applications and distribute it over the internet so that anyone can use your docker image.