Improve docker setup

- add .dockerignore to minimize build context
- add runtime ldap and sasl libraries
- add entrypoint script to allow initialization as root and drop
  privileges for gvaldap.sh
- add importlib-metadata dependency required by celery

.dockerignore

@@ -0,0 +1,11 @@
+**/*.pyc
+**/.coverage
+.env
+.git
+.gitignore
+.vagrant
+Vagrantfile
+change-vmdebootstrap-default-dhcp.sh
+coverage-report
+docs
+salt