Promtail: Update debian image and use a newer libsystemd

[slim-bean]

Fixes #2792, but really thanks to @fifofonix and others in that thread for doing all the hard work!

[fifofonix]

Apologies if I gave the impression that this was a fully worked out solution @slim-bean.

A key question here, and why I held off submitting a pull request myself, is whether this change will break others who are on older systemd versions and I think unfortunately there is a pretty good chance it will.

This is why I asked a question in the original issue regarding how we think we should be solving for this - should we be aiming for, for example, a version that builds to a specific docker tag aimed at newer systemd versions?

I will test the new version against an older pre-fedora 33 machine now, and get back to you if it crashes out, but even if it doesn't I'm not sure this is going to give me loads of confidence there won't be a regression somewhere.

[fifofonix]

Per comment on issue I have tested against older and newer systemd successfully which is good news. Still not 100% confident that the newer systemd API is backwardly compatible and will work on all old system configurations - but that is probably more of a comment on my lack of understanding of the details here.

[slim-bean]

Still not 100% confident that the newer systemd API is backwardly compatible

I'm less worried about this, the changes indicate that the journals created with newer files can't be read with older code (which we experienced) however they do not indicate any other kind of backward compatibility which I suspect they would have if this were expected.

Also some testing on our end and yours at least sanity checks that it still works. I am comfortable merging this and if someone upgrades and experiences problems at least the issues is quickly remedied by rolling back to the previous promtail version.

[cyriltovena]

LGTM

[owen-d]

Can we add a comment or two around this for future us? LGTM

[codecov-io]

Codecov Report

Merging #2957 (d6557fa) into master (4da505b) will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2957      +/-   ##
==========================================
- Coverage   61.88%   61.77%   -0.12%     
==========================================
  Files         182      182              
  Lines       14867    14867              
==========================================
- Hits         9201     9184      -17     
- Misses       4820     4837      +17     
  Partials      846      846              

Impacted Files	Coverage Δ
pkg/promtail/positions/positions.go	46.80% <0.00%> (-11.71%)	⬇️
pkg/promtail/targets/file/tailer.go	69.52% <0.00%> (-5.72%)	⬇️