[promtail] Allow parent charts to override registry hostname and credentials

[johnswarbrick-napier]

Reworked PR #2219 with a rebase and incremented minor version.

Add support for optionally overriding the registry hostname and credentials from a parent chart.

This is important for air-gapped installations which do not have direct access to the public internet and must use a private, internal container registry.

It also avoids having to modify the Promtail Helm chart directly.

FAO: @unguiculus and @Whyeasy

[johnswarbrick-napier]

Apologies, I didn't realise the README was generated automatically with helm-docs.

Have now reverted the change to README and hopefully this now passes the linting tests.

FAO: @unguiculus and @Whyeasy

[johnswarbrick-napier]

Hi @unguiculus and @Whyeasy

Sorry to chase, would it be possible to review and merge this PR please?

Really keen to get private registry supporting working for promtail

Thanks :)

[johnswarbrick-napier]

Hi @unguiculus and @Whyeasy - please could you review and approve this PR for PromTail?

Adding support for private registries will make it possible to deploy into "air-gapped" environments which do not have direct access to the public internet and must mirror everything into a private, internal container registry.

I have made similar changes to other projects in the Grafana and Prometheus Helm Chart repos. It's just Promtail which is remaining.

Thank you! :)

[johnswarbrick-napier]

Deployed and confirmed working - thank you @zanhsieh!