Add a Docker build CI step to reduce action build times

[ethanholz]

Using a pre-built container speeds up our builds and makes the base application more reproducible. Pushing this build to a container registry ensures we can achieve this.

[ethanholz]

Another potential solution here would be adopting a faster resolver such as uv, this may be a great option since uv does support a dependency lockfile which makes it easy for us to ensure the dependencies we use are the ones being deployed.