This removes existing dockerfiles and adds new ones. The removed
dockerfiles were originally used for building the testing images, but
now those images are generated by Docker Hub from the decred/dcrdocker
dockerfiles.
The new dockerfiles provide a way to build an image for running dcrd.
The main Dockerfile runs dcrd inside the same image used for building.
Dockerfile.alpine provides an alternative image that copies the built
files into an alpine based container, removing all build related content
and providing a securer and slimmer alternative.
