⬆️ Build Goji with Go modules, Go 1.19 and `GOAMD64=v3` (#7600)
* 🚚 Move template folder in src
This allows testing (building/running) outside of the Dockerfile.
* ⬆️ Bump Go from 1.14 to 1.19
* 🎨 Drop deprecated Go stuff from Dockerfile
* ⚡️ Set GOAMD64=v3 for AVX2, SSE4.2… instructions
See https://github.com/golang/go/wiki/MinimumRequirements#amd64
* ✨ Add `go.mod` in src (Go modules)