Improve E2E test run time by caching docker dependencies

[zarazan]

The github docker cache does not work between PRs and rebuilds dependencies way too often increasing the run time of the E2E test suite. The github cache has been disabled but a slightly more custom solution that maintains a cache of the docker layers that build dependencies would be helpful in decreasing run time.

Option 1: Intermediary docker image

• Github action that builds and pushes a new "builder" image each time the go.mod file is updated.
• In the e2e tests we will use that builder image, rather than official go image.

Option 2: Rely on docker-build command to determine which layers needs to be rebuilt

• Github action that builds and pushes a new full docker image each time a PR is merged into master
• Download the "main" docker image before running docker-build in the e2e tests

[robert-zaremba]

Thanks for adding this 👍