Use golang 1.11.10 and 1.12.5

Use the most recent versions of Go 1.11 and 1.12 in our build image: 1.11.10 and 1.12.5.

The motivation is that Go 1.11 has bugs in its Go modules support, which got fixed in Go 1.11.4. https://github.com/golang/go/issues/29278#issuecomment-447537558