Fix docker image tag with semantic version for releases

[askainet]

The current GHA workflow that builds and pushes docker images is configured to add a tag with the haystack version to the images when there is a new release, but this version tag is never created.

Proposed changes:

• Add the missing trigger to the docker_build GHA workflow so it runs also when creating tags in the git repository, i.e. on a new version release.
• Replace the docker image tag based on the raw git tag for a rule handling semantic versions.
• Update versions of actions docker/metadata-action and docker/build-push-action.

Status (please check what you already did):

• First draft (up for discussions & feedback)
• Final code
• Enable actions on your fork
• Added tests
• Updated documentation

[ArzelaAscoIi]

Nice! :) Could we somehow rerun this for v1.4.0 or should we manually publish an image ?

[askainet]

Nice! :) Could we somehow rerun this for v1.4.0 or should we manually publish an image ?

@ArzelaAscoIi I've manually pushed deepset/haystack-gpu:minimal-1.4.0 tag for the commit hash of v1.4.0 release 081b886