GUACAMOLE-408: don't exclude .gitignore in .dockerignore
GUACAMOLE-408: don't exclude .gitignore in .dockerignore

Because the .gitignore files are needed for configuring implicit
excludes for rat, .dockerignore must not exclude these files when
copying files into the image build context.

mike-jumper/incubator-guacamole-client,jmuehlner/incubator-guacamole-client,mike-jumper/incubator-guacamole-client,jmuehlner/incubator-guacamole-client,jmuehlner/incubator-guacamole-client,necouchman/incubator-guacamole-client,glyptodon/guacamole-client,mike-jumper/incubator-guacamole-client,lato333/guacamole-client,glyptodon/guacamole-client,necouchman/incubator-guacamole-client,jmuehlner/incubator-guacamole-client,lato333/guacamole-client,necouchman/incubator-guacamole-client,mike-jumper/incubator-guacamole-client,lato333/guacamole-client,glyptodon/guacamole-client,glyptodon/guacamole-client,necouchman/incubator-guacamole-client,glyptodon/guacamole-client,lato333/guacamole-client