Add `file` input to `build-push-action`

[cbini]

Exposing this configuration would allow Dagster repos w/ multiple code locations to use different Dockerfiles for each code location

https://github.com/dagster-io/dagster-cloud-action/blob/3eeebe9a0e7e346a854da59a91b3d4751bf7e198/actions/hybrid_branch_deploy/action.yml#L55-L66