Add coverage for Docker builds

[hugovk]

For #3606.

We're testing a number of Docker images on Travis CI (env: DOCKER=... in .travis.yml).

We're also testing them on GitHub Actions (.github/workflows/test-docker.yml, added in #4066) and Azure Pipelines (azure-pipelines.yml, #3670).

However, the Travis builds are tested with coverage which is sent to Codecov, but GitHub Actions and Azure Pipelines do not.

To speed up builds, it would be good to enable coverage on GHA or AP so we can stop testing Docker on Travis.

I don't have a strong preference for GHA or AP. We've started using GHA for Linux, macOS and Windows testing, so AP definitely would have more capacity free and would make things faster overall, and with eggs in more baskets. But GHA would mean there's fewer CI systems to maintain, and it feels a bit easier.