This introduces several changes to make the e2e test image logic more
robust:

- Rename IMAGE to GIT_SYNC_E2E_IMAGE to minimize accidental misuse
- Ensure that the image built by make container is the same as the image
  used by docker run
- Do not build the container if a custom image is specified

As an example, with the prior logic the following command always fails:
IMAGE="" make test
...
Unable to find image 'gcr.io/k8s-staging-git-sync/git-sync:latest' locally

Because the docker run command pulls a different image from the one
built by make container.

test: harden e2e image build logic

If a previous git invocation crashes, it is possible that an orphaned
lock file (e.g. shallow.lock) is left on the filesystem. This previously
caused git-sync to crash loop because the lock file is never deleted.

This change adds a check in sanityCheckRepo for the existence of a git
lock file. If the git lock file exists at this stage, then initRepo will
re-initialize the repository.

fix: recover when there is unreleased lock file

Signed-off-by: Liam Wyllie <[email protected]>

The GitHub app e2e test requires a GitHub app to be created and
installed, and also requires a few environment variables to be set.

This commit updates the GitHub action workflow by providing the
environment variables which can be set via GitHub Secret. GitHub
Secrests cannot start with `GITHUB_`. Hence, this commit prepends
`TEST_` to the env variables.

It also updates how GitHub app private key file is set. It can be set by
either `TEST_GITHUB_APP_PRIVATE_KEY` or
`TEST_GITHUB_APP_PRIVATE_KEY_FILE`.

For now we skip