EPIC2: Testground is reliable

[laurentsenta]

• [ ] #6
• [ ] #7
• [x] #8
• [ ] #9
• [ ] #11

[laurentsenta]

Stability issues:

• On panic (see https://github.com/laurentsenta/pl-github/issues/12#issuecomment-1109512140 the test should exit earlier, right now we wait for the test lifetime)
• Find a way to track the reasons for panics (see https://github.com/laurentsenta/pl-github/issues/12#issuecomment-1109512140)
• Fix the builder hangs when running go mod download even with direct proxy. Note that a go mod download with libp2p on a fresh goproxy (hetzner server) takes more than 10 minutes to complete
• LOCAL CI: Speed up builds by caching docker volumes (goproxy) & docker images (builds)