Default --root flag in container

This makes the container easier to use and allows docker volumes to be
used in a more obvious way.

Dockerfile.in

@@ -12,6 +12,38 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# HOW TO USE THIS CONTAINER:
+#
+# For most users, the simplest way to use this container is to mount a volume
+# on /tmp/git.  The only commandline argument (or env var) that is really
+# required is `--repo` ($GIT_SYNC_REPO).  Everything else is optional (run this
+# with `--man` for details).
+#
+# This container will run as UID:GID 65533:65533 by default, and unless you
+# change that, you do not need to think about permissions much.  If you run
+# into permissions problems, this might help:
+#
+#  - User does not mount a volume
+#    => should work, but limited utility
+#
+#  - User mounts a new docker volume on /tmp/git
+#    => should work
+#
+#  - User mounts an existing docker volume on /tmp/git
+#    => if the volume already exists with compatible permissions it should work
+#    => if the volume already exists with different permissions you can either
+#       set the container UID or GID(s) or you can chown the volume
+#
+#  - User mounts an existing dir on /tmp/git
+#    => set container UID or GID(s) to be able to access that dir
+#
+#  - User sets a different UID and git-sync GID
+#    => should work
+#
+#  - User sets a different GID
+#    => either add the git-sync GID or else set --root, mount a volume,
+#       and manage volume permissions to access that volume
+
 FROM {ARG_FROM}
 
 RUN apt-get update \
@@ -30,8 +62,26 @@ RUN echo "git-sync:x:65533:65533::/tmp:/sbin/nologin" >> /etc/passwd
 # which needs to be able to write to /etc/passwd.
 RUN chmod 0666 /etc/passwd
 
+# Add the default GID to /etc/group for completeness.
+RUN echo "git-sync:x:65533:git-sync" >> /etc/group
+
+# Make a directory that can be used to mount volumes and make it the default,
+# which makes the container image easier to use.  Setting the mode to include
+# group-write allows users to run this image as a different user, as long as
+# they use our git-sync group.  If the user needs a different group or sets
+# $GIT_SYNC_ROOT or --root, their values will override this, and we assume they
+# are handling permissions themselves.
+ENV GIT_SYNC_ROOT=/tmp/git
+RUN mkdir -m 02775 /tmp/git && chown 65533:65533 /tmp/git
+
+# Run as non-root by default.  There's simply no reason to run as root.
+USER 65533:65533
+
+# Setting HOME ensures that whatever UID this ultimately runs as can write to
+# files like ~/.gitconfig.
+ENV HOME=/tmp
+
 ADD bin/{ARG_OS}_{ARG_ARCH}/{ARG_BIN} /{ARG_BIN}
 
 WORKDIR /tmp
-USER 65533:65533
 ENTRYPOINT ["/{ARG_BIN}"]