Previous dockerfile (renamed to build.Dockerfile) allowed
building the project within dockerfile. However, the generated
image could not be uploaded to DockerHub. The new dockerfile
generates a minimal alpine image which can be used directly.
Also the produced binary is statically linked, so it can be
copied into other docker images without issues.

Older docker versions require -e flag to be present and the latest
docker version warns if password is not provided over stdin. This change
allows any of these behaviours to be specified using the same command
line flags.