Build Docker image as part of release process

[p-kimberley]

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.