add customChangelogUrl handlebar templating support

[gberche-orange]

What would you like Renovate to be able to do?

Many docker image packages are lacking the OCI label org.opencontainers.image.source to point to source and have their release notes in the created PR, as expected by Docker datasource.

The preferred approach is to contribute the LABEL instruction to each DockerFile project, (e.g. kudobuilder/kuttl#391) or suggest upstream projects to do the update in mass (e.g. crossplane/crossplane#3370)

However, this approach takes elapsed time to pay off.

As a complementary workaround, many docker images are named after their github source repo, therefore the customChangelogUrl can easily be programmatically constructed from the docker image {{depName}} if the customChangelogUrl pakcage rule would support handlebar templating

See related discussion at #18280 for further details

If you have any ideas on how this should be implemented, please tell us here.

Similar to other package rules handlebar templating support

Is this a feature you are interested in implementing yourself?

No

[rarkins]

Feel free to contribute to

renovate/lib/modules/datasource/metadata-manual.ts

Line 67 in 8ee28a4

docker: {

too in the meantime