Fixes to Docker GitHub Actions workflow

[AFCMS]

Two fixes to the Docker image workflow.

• Fix the workflow run on tags (was untested in my first PR, my bad)
• Do not use Docker cache for tag builds, also add a parameter to workflow_dispatch to allow building without cache

To do

This PR is Ready for Review.

How to test

Get these commits in the master branch of a Minetest fork.

Try to run the workflow with workflow_dispatch, with and without the "Use build cache" checkbox.

Push a tag to the fork, see the workflow run in the Actions tab (unlike with the current workflow).

For these runs check the parameters of the Build and push Docker image step (in the first folding section of the log), see the no-cache parameter. It should be true on tags, false on normal push, false on PRs, true or false depending of the workflow_dispatch input.

This PR fixes the build on tags, so it should probably be merged for 5.9.0 (yet the official image could be build after the release using workflow_dispatch)

[sfan5]

lgtm