Build Docker image as part of release process

The Docker image should consist of a minimal web server (e.g. based on the `nginx-unprivileged` image) that serves static `stroom-docs` files.

The intent is for this image to be deployed to air-gap environments where access to the public `stroom-docs` site is unavailable.

Images should be uniquely tagged with each release, for instance by incorporating the release number into the tag name.

Start by locating the repository's release process and the generated static stroom-docs output. Determine where a Docker build and release-specific image tag can be integrated, using an nginx-unprivileged-based image to serve the files. Done means the image is minimal, serves the documentation for air-gapped deployment, and receives a unique tag for each release.

Written by the indexing model from the issue text.