Run build in test/ci branches

[ggiraldez]

The script refuses to build unless it's being run from a specific version tag or the master or release/xx branches, because it can't define the DOCKER_TAG variable. It would be useful to be able to run the script on test branches. For example, it's common practice to create a ci/fix branch to fix CI related issues. The built Docker tag could be a throw-away or some generic one. The purpose of the build is to check that everything works end-to-end.

[matiasgarciaisaia]

@ggiraldez I think it's not the same thing, but please check if #8 helps somehow.

If not, can you clarify a bit the desired behaviour?

[ggiraldez]

I can think of two use cases:

• testing that the Docker image builds, but not necessarily pushing the image to the registry; IIRC this was the original intent of the issue
• building a "preview" image to e.g. test a feature in Rancher staging, nicely addressed by your PR