Add Alpine Linux variant

[michaelbaudino]

This commit:

• changes Dockerfile to install dependencies with either apk (Alpine) or apt-get (Debian)
• changes entrypoint executor from /bin/bash to /bin/sh (since Alpine does not ship with bash by default)
• changes gosu-entrypoint to properly create user/group on either Alpine or Debian
• ensures expected mountpoint exist at least as empty directories
• changes Travis build script to build -alpine images
• moves Travis build script to an external file