Add Dockerfile support

[pataquets]

Add Dockerfile to enable image building. Useful for development and tests under Docker workflow.
Using the official Golang image, latest tag. More info at https://hub.docker.com/_/golang/

Install package dependencies, add files, set working dir and build. Nothing too fancy.

Build:

$ docker build -t dms .

Run:

$ docker run --net=host --rm -it -v /some/host/dir/:/var/lib/dms/ dms -path /var/lib/dms/ [other options...]

FYI, there's a still quicker to test, already built image on my Docker Hub. Test it by running:

$ docker run --net=host --rm -it -v /some/host/dir/:/var/lib/dms/ pataquets/dms-src -path /var/lib/dms/ [other options...]

Using --rm instead of -d makes the container not go background and it to be deleted after stop. Should stop by CTRL+C'ing it.
Choose some existing media dir from your host filesystem to mount inside the container (-v switch).
Uses the --net=host switch to access the host's network stack instead of using container port mappings, for simplicity.

Optional improvement to come (maybe in another issue):

• Create an 'official', based on your repo, automated build at Docker Hub for the image: https://docs.docker.com/docker-hub/builds/ . Just requires a free Docker Hub account and a following a quick 'Create automated build' process. I'll be happy to help on it, if needed.