Dockerfile updates (#77)

* Added VERB environment variable for enabling verbosity in the Docker container
* pin the versions for golang and alpine at their current latest
* move the entrypoint script to a separate file, removed EXPOSE command from Dockerfile

support/entrypoint.sh

@@ -0,0 +1,2 @@
+#!/bin/sh
+exec su-exec ${UID}:${GID} /gossa -h ${HOST} -p ${PORT} -k=${SKIP_HIDDEN_FILES} -ro=${READONLY} --symlinks=${FOLLOW_SYMLINKS} --prefix=${PREFIX} --verb=${VERB} ${DATADIR}