Latest version placeholder

[francislavoie]

I originally asked about this here #1802 (comment) but I didn't get an answer, so I figured I'd open an issue instead to discuss it.

Is there any way to dynamically replace this <version> with a placeholder? I dug through the scripts in this repo and I didn't find any relevant placeholders.

Ideally, I'd like a %%LATEST_VERSION%% placeholder that would basically be the version number that the latest tag shares.

I've already gotten a question from a user recently who is new to Docker asking "do I need to replace <version>?" so I think this should be improved.

I realize that this is probably non-trivial because the latest tag doesn't directly map to a specific version because of how tagging works. Maybe there's a way to hook into the docs generator to provide additional placeholders? We could write a simple script that extracts the relevant info based on some heuristic.

[yosifkit]

Yeah, it is a bit hard to find which tag is the most equivalent to latest (or generally, getting the "most specific" tag from any less specific tag). While we usually order our tags from most specific to least, not all images follow this unspoken convention. So, we could guess by taking the first tag in the Tags: list, but should the docs really be changed for every minor version bump?

Then there is the issue of latest being a "Shared Tag" and so can appear more than once in the library file (like python on line 38, line 54, and line 61) and so even bashbrew list doesn't give us a canonical answer:

$ bashbrew list python:latest
python:3.9.0-buster
python:3.9-buster
python:3-buster
python:buster
python:3.9.0
python:3.9
python:3
python:latest
python:3.9.0-windowsservercore-ltsc2016
python:3.9-windowsservercore-ltsc2016
python:3-windowsservercore-ltsc2016
python:windowsservercore-ltsc2016
python:3.9.0-windowsservercore
python:3.9-windowsservercore
python:3-windowsservercore
python:windowsservercore
python:3.9.0
python:3.9
python:3
python:latest
python:3.9.0-windowsservercore-1809
python:3.9-windowsservercore-1809
python:3-windowsservercore-1809
python:windowsservercore-1809
python:3.9.0-windowsservercore
python:3.9-windowsservercore
python:3-windowsservercore
python:windowsservercore
python:3.9.0
python:3.9
python:3
python:latest
$ bashbrew list --uniq python:latest
python:3.9.0-buster
python:3.9.0-windowsservercore-ltsc2016
python:3.9.0-windowsservercore-1809

I think we end up just putting a major version and then eventually updating the doc when that version is no longer supported.