fix(helm): support global registry override for sidecar

[Scrumplex]

What this PR does / why we need it:

Since #19246 the sidecar image would resolve to <registry-mirror>/docker.io/kiwigrid/k8s-sidecar when a global registry mirror was defined.

This commit handles the registry properly, the same way as all the other docker hub images.

Which issue(s) this PR fixes:

Special notes for your reviewer:

I have tested this using helm template. Without a global registry the output is unchanged. With a global registry, it is now resolved properly.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[Scrumplex]

Sorry that it took so many tries to update the docs

For some reason the Makefile step didn't work for me on NixOS.

$ make -BC docs sources/setup/install/helm/reference.md
make: Entering directory '/home/scrumplex/Projects/loki/docs'
podman run --rm --volume "/home/scrumplex/Projects/loki:/helm-docs" -u "$(id -u)" "docker.io/jnorwood/helm-docs:v1.11.0" \
	-c /helm-docs/production/helm/ \
	-t reference.md.gotmpl \
	-o reference.md
podman run --rm --volume "/home/scrumplex/Projects/loki:/helm-docs" -u "$(id -u)" "docker.io/jnorwood/helm-docs:v1.11.0" \
	-c /helm-docs/production/helm/ \
	-t README.md.gotmpl \
	-o README.md
mv "../production/helm/loki/reference.md" "sources/setup/install/helm/reference.md"
time="2025-11-04T20:08:13Z" level=info msg="Found Chart directories [loki]"
time="2025-11-04T20:08:13Z" level=info msg="Generating README Documentation for chart /helm-docs/production/helm/loki"
time="2025-11-04T20:08:13Z" level=warning msg="Could not open chart README file /helm-docs/production/helm/loki/README.md, skipping chart"
time="2025-11-04T20:08:13Z" level=info msg="Found Chart directories [loki]"
time="2025-11-04T20:08:13Z" level=info msg="Generating README Documentation for chart /helm-docs/production/helm/loki"
time="2025-11-04T20:08:13Z" level=warning msg="Could not open chart README file /helm-docs/production/helm/loki/README.md, skipping chart"
mv: cannot stat '../production/helm/loki/reference.md': No such file or directory
make: *** [Makefile:16: sources/setup/install/helm/reference.md] Error 1
make: Leaving directory '/home/scrumplex/Projects/loki/docs'

Adding BUILD_IN_CONTAINER=false fixed the issue for me (after supplying helm-docs)