Tag official releases

[alanbchristie]

The backend image is used as a FROM in downstream container images like the fragalysis-loader (https://github.com/xchem/fragalysis-loader) and fragalysis-stack (https://github.com/xchem/fragalysis-stack) but, at the moment we only publish a "latest" image.

Currently, if you introduce a new migration or behaviour change (for example) - and you re-build an older stack or loader (say 1.0.7) - then the stack or loader you build may contain unexpected migrations or behaviour changes.

Recommended pattern:

When a stable backend is available...

1. Create a GitHub tag (or release) in the backend repo (like 1.0.0)
2. Travis will push the image automatically to Docker (i.e. as xchem/fragalysis-backend:1.0.0)
3. When the backend has been pushed update each downstream repo's Dockerfile and replace the ARG BE_IMAGE_TAG="latest" with ARG BE_IMAGE_TAG="1.0.0"