Fix release workflow

[steven-sheehy]

What this PR does / why we need it:

• Re-enables the release workflow disabled in release stage is broken, removing for now #647
• Main fix was to add ignore branches since by default tags only confusingly builds branches as well
• Docker tag is hardcoded to master instead of tools/image-tag since the latter uses a prefix of HEAD for tags which does not exist in Docker hub. This should be fine for now unless you guys ever tag from a different branch
• Change helm chart to publish only on a tag
• Fix loki-docker-driver not using IMAGE_PREFIX, making it impossible to run in my own CIrcleCI

Which issue(s) this PR fixes:

Special notes for your reviewer:
You can see proof that it works here:

https://circleci.com/gh/steven-sheehy/loki/524
https://github.com/steven-sheehy/loki/releases

Checklist

• Documentation added
• Tests updated

[cyriltovena]

Looks good @slim-bean I defer to you here.

[slim-bean]

I think we covered the spirit of this PR with the latest overhaul of the circle config and makefile, so I'm gonna close this PR. Please let me know if we are missing anything @steven-sheehy

[steven-sheehy]

It still publishes a helm chart on every commit and not on a tag only. This was the main purpose of the PR.

[slim-bean]

I created a separate issue to improve the helm chart release process #852 which also identifies some challenges we face around creating a release PR and our builds.