Update builder image to Go 1.13

Due to force-push in github.com/labbsr0x/goh none of the Caddy versions can be built with Go 1.12 (missing commit hash). Go 1.13, which uses modules proxy, can build Caddy latest versions, as it will find this commit on public proxy.

I suggest you to update builder image to `golang:1.13-alpine`, so all builds will work again.