Add Dockerfile

[Daviey]

This change adds a Dockerfile which builds the binary in an Alpine linux image and copies it into a small Alpine image.

It requires an environment variable of HOSTNAME, and takes an optional variable of WORKGROUP (and defaults to "WORKGROUP").

This can be tested with:

docker build -t ${USER}/wsdd2
docker run -ti -e HOSTNAME=MYSERVER ${USER}/wsdd2

Signed-off-by: Dave Walker (Daviey) email@daviey.com

[Andy2244]

sure looks usefully, thanks