DevOps Classroom Series – 31/Oct/2020

Building Docker Images

• Building Docker Images can be done in two ways
  • Create a container, do the manual installation & create a docker image from container.
    • This approach is not sensible as the image creation process is manual & cannot be version controlled
    • Changes are difficult to handle
  • Create a docker container by writing a Dockerfile
    • This approach helps in maintaining versions of every change which you do
    • Changes will be simple as it all about changing one file & create a new version
• Dockerfile is a simple plain text with a set of INSTRUCTIONS
• Workflow:
• Basic Steps involved in building a docker image
  • choose the right base image
  • add steps to install/configure your application
  • add information of ports required etc
  • add step which should be executed to run your application
• This is exactly what we do to in terms of Dockerfile. In Dockerfile we have lot of instructions. Each instruction performs some activity. So lets look at Dockerfile instructions
• Docker image has the following naming conventions <image-name>:<tag>
• Refer Here for the official documentation.
• Lets try to create a simple dockerfile
• Refer Here for changes
• Lets try to build a image
• Now lets try to build our docker image with name my-helloworld
• When we build docker image an image id, image name & image tag are associated with a image
• Now lets use an instruction LABEL to add metadata Refer Here for the changeset
• The inspection result is

[
    {
        "Id": "sha256:ba68f57b7991745a56bbc696ab0f15aa81e7e712dbf5702ebc67044e41f8dd73",
        "RepoTags": [
            "my-helloworld:latest"
        ],
        "RepoDigests": [],
        "Parent": "sha256:9932ce15331bc6adaef6ccf95a8ce0fdaa389714097b2adad4e68ee2dae76294",
        "Comment": "",
        "Created": "2020-10-31T13:24:43.128925375Z",
        "Container": "3fe99f6b027b8d97102f981384539046432bd6c09599353de86088896b569306",
        "ContainerConfig": {
            "Hostname": "3fe99f6b027b",
            "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": [
                "/bin/sh",
                "-c",
                "#(nop) ",
                "LABEL organization=learning thoughts"
            ],
            "ArgsEscaped": true,
            "Image": "sha256:9932ce15331bc6adaef6ccf95a8ce0fdaa389714097b2adad4e68ee2dae76294",
            "Volumes": null,
            "WorkingDir": "",
            "Entrypoint": null,
            "OnBuild": null,
            "Labels": {
                "author": "khajaibrahim",
                "organization": "learning thoughts"
            }
        },
        "DockerVersion": "19.03.13",
        "Author": "",
        "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": [
                "/hello"
            ],
            "ArgsEscaped": true,
            "Image": "sha256:9932ce15331bc6adaef6ccf95a8ce0fdaa389714097b2adad4e68ee2dae76294",
            "Volumes": null,
            "WorkingDir": "",
            "Entrypoint": null,
            "OnBuild": null,
            "Labels": {
                "author": "khajaibrahim",
                "organization": "learning thoughts"
            }
        },
        "Architecture": "amd64",
        "Os": "linux",
        "Size": 13336,
        "VirtualSize": 13336,
        "GraphDriver": {
            "Data": {
                "MergedDir": "/var/lib/docker/overlay2/cb7f79e0d542a174d630cb7517f46775b6e117f312c9e5b6a6aca1c469964a43/merged",
                "UpperDir": "/var/lib/docker/overlay2/cb7f79e0d542a174d630cb7517f46775b6e117f312c9e5b6a6aca1c469964a43/diff",
                "WorkDir": "/var/lib/docker/overlay2/cb7f79e0d542a174d630cb7517f46775b6e117f312c9e5b6a6aca1c469964a43/work"
            },
            "Name": "overlay2"
        },
        "RootFS": {
            "Type": "layers",
            "Layers": [
                "sha256:9c27e219663c25e0f28493790cc0b88bc973ba3b1686355f221c38a36978ac63"
            ]
        },
        "Metadata": {
            "LastTagTime": "2020-10-31T13:24:43.139925002Z"
        }
    }
]

• Now lets try find history of image

Lets try to build a gameoflife docker image

• Select base image => tomcat with version 8 of java
• download the file from url into webapps folder
• This application runs on 8080 port
• running this app is running tomcat
• Now lets create a Dockerfile Refer Here for the changes
• Lets try to create continer with gameoflife image docker container run -d -P gameoflife:1.0

Lets try to build one more image for spring pet clinic

• This application just requires java to be present.
• Copy the jar file and when you want to run the application execute command java -jar <package>.jar
• Over here we need to worry about how to start the application.
• Now lets look at steps
  • Try to find a image with java 8 installed
  • Copy the jar file into some path
  • Expose port 8080
  • While starting container execute the command java -jar
• Lets use FROM, LABEL instructions as usual for copying the jar file lets use wget instruction and for running the command during container starts lets use CMD instruction
• Refer Here for the changes

From the above 3 examples (Lessons Learnt)

• While building images We need to consider
  • picking right base image
  • exposing the ports which application uses
  • commands to install/configure the application
  • commands to start the application