Use explicit image paths

To make it clear that docker hub is but one of many possible registries,
update our usage of FROM and image: lines to include docker.io in the
path.

There are a few other FROM lines for the gitea images which are handled
in a separate stack.

Change-Id: I6fafd5f659ad19de6951574afc9a6b6a4cf184df

docker/python-base/Dockerfile

@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM python:3.7-slim
+FROM docker.io/library/python:3.7-slim
 
 RUN apt-get update \
   && apt-get install -y dumb-init libjemalloc2 \