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.