Building Docker Images from remote repositories

I recently wanted to build a Docker Image directly form a source repository without having to clone the repository locally before building it. This turned out to be such a neat little trick that I thought it was worth sharing.

Inspecting what the docker build command had to offer with regards to building repote repositories yield an interesting parameter named URL.

$ docker build --help
> Usage: docker build [OPTIONS] PATH | >>> URL <<< | -
> Build a new image from the source code at PATH

Consulting the online build documentation confirmed my suspicion that this was in fact an option to build a Docker Image directly form a remote repository.

The URL parameter can specify the location of a Git repository; the repository acts as the build context.

The URL parameter also has the optional possibilities to specify a which branch, tag, or commit sha to build, as well as the location (directory) of the Dockerfile you want to build.

$ docker build https://github.com/docker-library/hello-world.git#master:/
> Sending build context to Docker daemon 132.1 k
> Sending build context to Docker daemon
> Step 0 : FROM scratch
>  --->
> Step 1 : COPY hello /
>  ---> a1e6124c9115
> Removing intermediate container b27bf9c2271f
> Step 2 : CMD /hello
>  ---> Running in 80cd51769b8e
>  ---> 3ec9c8c47f66
> Removing intermediate container 80cd51769b8e
> Successfully built 3ec9c8c47f66

Epilogue

I later realized that the repository is cloned locally behind the scenes which makes that sense when you think about authentication, vpn etc to private repositories.

This command runs in a temporary directory on your local host. After the command succeeds, the directory is sent to the Docker daemon as the context. Local clones give you the ability to access private repositories using local user credentials, VPNs, and so forth.

A Node.JS Example

If you are interested in how to build a remote repository using the Docker Remote API with the dockerode Docker client library for Node.JS, this is how you do it:

```javascript const options = { t: ‘docker-library/hello-world’, remote: ‘https://github.com/docker-library/hello-world.git’, };

docker.buildImage(null, options, function(err, res) { if (err) { throw err; }

res.on(‘data’, function(data) { console.log(data.toString()); }); });