Fix the build in Docker after enabling go modules

Enabling go modules requires that GO111MODULE is set, so
we set it as an environment variable in the Dockerfile.

Also, go-bindata is meant to be installed globally, so we
force the install before enabling Go modules. Also, we update
Go to 1.12 to fix a couple module builds.
1 job for master in 2 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
passed #290