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.
This adds the first version of a Dockerfile, built on an Alpine Linux variation of the golang image, as well as a Docker Compose file that spins up both a MariaDB and the instance. It also updates the README with instructions on how to get Write Freely running with this setup.