Exclude some Dockerfiles from Dependabot upgrades
Exclude some Dockerfiles from Dependabot upgrades

Dependabot errors out on one of the root Dockerfiles (because
public.ecr.aws doesn't support an API it requires, see
https://github.com/dependabot/dependabot-core/issues/4212). To avoid
this, I've changed the dependabot configuration to only upgrade Vale.

This is a follow-up to https://github.com/buildkite/docs/pull/1538.

buildkite/docs,buildkite/docs,buildkite/docs,buildkite/docs