Build esp-v2 with GitHub Actions?

[timburks]

To make it easier to test experimental changes to the proxy, I configured a GitHub Action to build it.

Would the team like to have this in the main repo? A downside is that it takes ~1h 45min to run, which is both painfully slow and burns a lot of GitHub Action minutes if it's done on every checkin, but it could be configured to just run nightly or weekly. Also, I didn't see a way to do this with the existing Dockerfiles, so I wrote this one.