Clarify support policy (for Alpine)?

[Schweinepriester]

Problem

Specific: We were using node:18-alpine3.17 and surprised to find out, when we wanted to use the latest node version 18.20.2, that the tag had last been updated 5 month ago (src), because support was removed in #2007, even though Alpine 3.17 itself is still supported until 2024-11-22 (src).
Following the link chain in the PR to previous PRs it seems the policy has been "support last two minor versions" (of at least Alpine) for some time, but this doesn't seem to be transparent in the docs (or I missed it).
So, currently it's not clear when we need to update the tag to be on a version which still receives updates.

Abstract: Support policy unclear (at least for Alpine)

Solution

Make apparent support policy "last two minor versions" or something alike transparent in the docs, probably in the README.

Alternatives to Consider

1. Do nothing 😢
2. Support images until their respective EOL
   • more desirable from my perspective
   • more effort, perhaps not feasible

[nschonni]

Agree that there should be a note, along with #2011.
We only support 2 at the recommendation of the Official Images repo as there is a lot of build required for these images, and there is no upstream support or binaries for Alpine that would reduce that workload if more were added back