Add debug info about base image sha

In order to debug issues, knowing what version of a base docker image is
in use can be helpful. Grab it and print it out.

Change-Id: I741b56bf4f8f8d1edd7e40da415b26c2e3dd13d7

pbrx/container_images.py

@@ -66,6 +66,12 @@ class ContainerContext(object):
                 host=os.path.expanduser('~/.config/pip/pip.conf'),
                 guest='/root/.config/pip/pip.conf'))
         self.create()
+        log.debug(
+            "Used base image {base} at sha {sha}".format(
+                base=self._base,
+                sha=sh.docker.images('-q', self._base).strip(),
+            ))
+
         self._cont = sh.docker.bake("exec", self.run_id, "sh", "-c",
                                     _truncate_exc=False)