Add entrypoint.sh to abstract around metadatad binary.

pkg/docker/Dockerfile-xenial

@@ -1,6 +1,8 @@
 # To use directly, run: docker build --build-arg package=<pkg>.xenial.deb -t <tag> -f Dockerfile-xenial
 FROM ubuntu:xenial
 
+EXPOSE 8000
+
 ARG version=0.0.19-1
 ARG package=https://storage.googleapis.com/stackdriver-container-alpha/deb/xenial/stackdriver-metadata_${version}.xenial.deb
 ADD ${package} /stackdriver-metadata.deb
@@ -9,7 +11,7 @@ RUN apt-get update \
     && apt-get install -f -y \
     && rm -rf /var/lib/apt/lists/* \
     && rm -rf /stackdriver-metadata.deb
+COPY entrypoint.sh /entrypoint.sh
 
-CMD /opt/stackdriver/metadata/sbin/metadatad
-
-EXPOSE 8000
+ENTRYPOINT ["/entrypoint.sh"]
+CMD ["/opt/stackdriver/metadata/sbin/metadatad"]

pkg/docker/entrypoint.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+set -e
+
+# This docker image supports sending either a flag or a command as the docker
+# command. When a flag is sent, it will be passed on to the metadata agent 
+# process. Anything else will be interpreted as the command to be run.
+#
+# Passing a flag.
+# $ docker run -it {image:tag} -v
+#
+# Passing a command.
+# $ docker run -it {image:tag} /bin/bash
+#
+# Default behavior uses CMD defined in Dockerfile.
+# $ docker run -it {image:tag}
+
+if [ "${1:0:1}" = '-' ]; then
+  set -- /opt/stackdriver/metadata/sbin/metadatad "$@"
+fi
+
+exec "$@"