Use latest docker image tags for new releases only

[mumoshu]

#300 made me realize that some people use latest tag for the controller image in production deployments. I believe changing our workflow to update latest tag only on new releases makes things a bit more reliable for them.

Currently latest is automatically updated on each new commit in the main branch. As I usually do some additional manual tests before releasing a new version of the controller, updating latest only on new releases result in the controller deployments get updated only with manually tested releases.

From a maintainer's perspective, reports like #300 is SUPER HELPFUL as I can notice/confirm that there's indeed issue(s) that I missed while reviewing or limited manual tests I often do before merging PRs.

So, I'd like to introduce another tag, canary for one that is updated on every commit.

[donovanmuller]

@mumoshu related to this, is there a reason that the Helm chart uses tag: "latest" and not simply a blank value to pick up the appVersion which would be much more stable?

[mumoshu]

@donovanmuller Ah, good catch! Probably we'd better just use appVersion and make the default tag empty. I had written tag: "latest" without thinking much.

[callum-tait-pbx]

See my comments on my PR for more details but would it not be better to set the default to latest in the values.yaml that is bundled with the chart, the Helm chart AppVersion really doesn't mean anything for this project. If we change the release workflow as suggested in this issue so that latest is only updated when we are confident it is stable then we shouldn't have a problem in general?