COPY --from= can specify external images. Add support to renovate them.

Adds field prettyDepType available for templates, currently used for npm only. Allows for PR titles like “Update devDependency left-pad to v1.3.0” instead of default “Update dependency left-pad to v1.3.0”.

To enable in PR titles: add this configuration: `"commitMessageTopic": "{{prettyDepType}} {{depName}}”`

This is not enabled by default as otherwise it could lead to the reopening of previously manually-closed PRs.

Closes #2371, Closes #1863

Closes #2374

If the dependency already is the latest version, filteredVersions is
empty here, and then the early exit skips over the pinning code

fix(docker): resolve relative URLs from registry

The default prefix is library/. Using amd64/ as the prefix created weird
issues with pinning to digests that matched no images.

Supports github.com only

Related to #2323 and #1925