Provide updated 6.0.X backend images with dependencies from 6.1 backend

[fredvd]

For Plone 6.1 the images (starting with 6.1.0rc1) we have updated the image OS, Python and several dependencies. $162

@mauritsvanrees asks if we can also update the 6.0.X line.

This can potentially break FROM AS usage in project Dockerfiles that depend on plone-backend 6.0.X . When integrators check semantic versioning and think 6.0.14 -> 6.0.15 is just a patch version.

We can follow plone-frontend with yarn and pnpm flavours. The downside is extra maintenance in the 6.0.X backend building, but I don't think the overhead will be that big.

Should we create a 6.0.15 and 6.0.15-ir2. (image refresh). How should we call that label then?

This is also interesting for upgrades: first update to 6.0.X latest with the image refesh. Then upgrade to 6.1 where 'only' the Plone code changes, but the os/python/dependencies stay stable.

[davisagli]

I don't like the idea of a patch version update to the image which updates the underlying OS and Python. That sounds like it will break someone.

Releasing a new image with a different suffix sounds okay. Shall we follow our underlying images (the Python and Node ones) and use a suffix based on the OS flavor? i.e. -bookworm