The options used were wrong in two ways: firstly they were not enforced
as a "choice" (i.e. invalid input valudes could be provided without
error) and one of the options was listed as `gh` when we passed it as
`gha` from ci.yml.
"Fix" this by removing the choice altogether but sanity-testing the
input value against an expected list using a GHA "warning" to notify of
unknown inputs.
When using `docker buildx build` in conjunction with the `gha` backend
cache type, it's important to specify the URL and TOKEN needed to
authenticate.
On Cirrus runners this is working with only `ACTIONS_CACHE_URL` and
`ACTIONS_RUNTIME_TOKEN`, but this is not enough for the GitHub backend.
Fix this by exporting all `ACTIONS_*` variables.
This fixes cache restore/save on forks or where GH-hosted runners are
being used.
Another action to reduce boilerplate in the main ci.yml file.
This action will set up a docker builder compatible with caching build
layers to a container registry using the `gha` build driver.
It will then configure the docker build cache args.
