feat(RELEASE): allow overriding build.sh

[akloeckner]

I'm experimenting with GitHub workflows and found the current setup has some minor flaws that prevent building the images, if the current release is already compiled and available on Docker hub. This is necessary, e.g., when requesting a build manually, or if only the Dockerfilehas changed. This PR fixes those issues in the following way:

• add ARG for RELEASE_URL
• make build.sh prepare .tags even if tag exists
• add output release_url to be used by github workflows

build.sh is still aborted before changing Dockerfile, so people can run it locally without modify the git repo. .arg may be changed, because it is .gitignored.

Here's an example of how to use the changed script in a GitHub workflow: https://github.com/akloeckner/TileBoard-docker/blob/26e167b4d57bbca1913f75e3f100aa22babf3973/.github/workflows/release.yml#L32

There should be no impact on current build setups. (Other than a somewhat cryptic echo of the release URL.)

[fbrinker]

Thank you, good idea :)