Cloning source code of a docker image

We all know that docker is a great tool to build and deploy applications. However, it is rather difficult to get the source code of a 3rd party image as a developer. In this guide, I will attempt to clone the source code of a downloaded image from a remote server.

There are some things to keep in check in case you come across such:

Inspecting a docker image:

sudo docker image inspect <IMAGE_NAME>

This command will provide the image configuration in JSON with one crucial detail to look for i.e., where the actual code of the image resides within it. Look out for WorkingDir under ContainerConfig and that will be the root of the source code being run by the image.

For Example:

[
    {
        "Id": "...",
        "RepoTags": [
            "..."
        ],
        "RepoDigests": [
            "..."
        ],
        "Parent": "",
        "Comment": "",
        "Created": "2020-11-04T16:28:13.6789081Z",
        "Container": "...",
        "ContainerConfig": {
            "Hostname": "cd849c8982e0",
            "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",
                "NODE_VERSION=14.15.0",
                "YARN_VERSION=1.22.5",
                "NODE_ENV=production"
            ],
            "Cmd": [
                "/bin/sh",
                "-c",
                "#(nop) ",
                "CMD [\"node\" \"/var/apps/app-name/main.js\"]"
            ],
            "Image": "...",
            "Volumes": null,
            "WorkingDir": "/var/apps/app-name", // <--- THIS LINE
            "Entrypoint": [
                "docker-entrypoint.sh"
            ],
            "OnBuild": null,
            "Labels": {}
        },
        "DockerVersion": "19.03.12",
        "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",
                "NODE_VERSION=14.15.0",
                "YARN_VERSION=1.22.5",
                "NODE_ENV=production"
            ],
            "Cmd": [
                "node",
                "/var/apps/app-name/main.js"
            ],
            "Image": "...",
            "Volumes": null,
            "WorkingDir": "/var/apps/app-name",
            "Entrypoint": [
                "docker-entrypoint.sh"
            ],
            "OnBuild": null,
            "Labels": null
        },
        "Architecture": "amd64",
        "Os": "linux",
        "Size": 932787935,
        "VirtualSize": 932787935,
        "GraphDriver": {
            "Data": {
                "LowerDir": "...",
                "MergedDir": "...",
                "UpperDir": "...",
                "WorkDir": "..."
            },
            "Name": "overlay2"
        },
        "RootFS": {
            "Type": "layers",
            "Layers": [
                "...",
                "...",
                "...",
                "...",
                "...",
                "...",
                "...",
                "...",
                "...",
                "...",
                "...",
                "..."
            ]
        },
        "Metadata": {
            "LastTagTime": "0001-01-01T00:00:00Z"
        }
    }
]

Cloning a docker image

# Create a new temporary image from the existing docker image
$ sudo docker create --name="tmp_$$" <IMAGE_NAME:TAG>

# Export the temporary image to a tarball
$ sudo docker export tmp_$$ > image.tar

# Delete the temporary image
$ sudo docker rm tmp_$$

# Let's use sftp to download the tarball to our local machine
$ sftp -i <KEY> <USERNAME>@<SERVER_NAME>
$ get image.tar

# Now once you have the tarball downloaded to you local machine, extract and navigate to the `WorkingDir` highlighted earlier, which here was `/var/apps/app-name/`
$ cd /Downloads/<EXTRACTED_IMAGE_FOLDER_NAME><WorkingDir>

And there you have it, the source code of a 3rd party image.

Things to consider before attempting this:

• sftp will use your local bandwidth to download any files from the remote server, so a Wi-Fi/Broadband connection is suggested for this as the docker images tend to be rather big in size and will take a considerable amount to download, therefore.
• Add sudo before the docker commands in case without sudoer it doesn't work.

Comments

[Chris Frewin]

Great post, I used it to figure out what was actually running behind some of the kubernetes tutorials I've been doing. I've abstracted it to a shell script here:

gist.github.com/princefishthrower/...

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