Keep latest tags for branches

[skovhus]

We recently introduced a change to not not tag images on non-main branches with "latest". This avoids cache breaking as all branches will not all push the "latest" image.

At least locally we shouldn't do this as the latest tag is usually used for local development in Docker compose files.

Ideas:

1. I think most CIs adds a CI=1 environment variable (we could use that)
2. Or be explicit with an additional argument (but might be confusing)

[corradio]

Here's an additional proposal:
Right now, it seems like the "latest" tag is always used as cache key.
If the cache-from directive supports having multiple cache entries as potential input, then my suggestion would be to use cache-from <image>:master, cache-from <image>:<current-branch>, which, in theory, should always either cache from current branch or from master (and never from latest).

Hope it makes sense - else, happy to clarify.

[skovhus]

That is a great idea, and it seems to work. But I would leave this out for in a WIP PR now and observe how CI behaves.