Align mini-mirror Dockerfile args with build.sh

The mini-mirror build.sh script has somehow ended up out of sync with
the Dockerfiles, and is not passing the correct build-args to allow base
image overrides.

This change aligns with the standard of using BUILD_FROM (ubuntu) for
the build stage and FROM (nginx) for the final stage.

Relevant earlier changes:
0: https://review.opendev.org/714002
1: https://review.opendev.org/679315

Change-Id: I844efa669c0f11004a44f20f6725338e1d2699b6

mini-mirror/Dockerfile.ubuntu_bionic

@@ -12,9 +12,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-ARG FROM_UBUNTU=ubuntu:18.04
-ARG FROM_NGINX=nginx
-FROM ${FROM_UBUNTU} as aptly
+ARG BUILD_FROM=ubuntu:18.04
+ARG FROM=nginx
+FROM ${BUILD_FROM} as aptly
 
 ARG APTLY_CONFIG_PATH=etc/aptly.conf
 ARG MIRROR_SOURCE_FILE=mini-mirror-sources.yaml
@@ -40,7 +40,7 @@ COPY tools/publish_snapshots.sh /opt/publish_snapshots.sh
 
 RUN /opt/publish_snapshots.sh "${RELEASE_SIGN_KEY_PASSPHRASE}"
 
-FROM ${FROM_NGINX}
+FROM ${FROM}
 
 ARG APTLY_SNAPSHOT_DIR=/srv
 

mini-mirror/build.sh

@@ -45,8 +45,8 @@ APTLY_REFSPEC=${APTLY_REFSPEC:-"allow-custom-codename"}
 
 docker build -f "${PROJECT_PATH}"/Dockerfile."${DISTRO}" --network=host \
   -t "${REGISTRY_URI}""${IMAGE}":"${VERSION}"-"${DISTRO}""${EXTRA_TAG_INFO}" \
-  ${BASE_IMAGE_UBUNTU:+--build-arg FROM_UBUNTU=${BASE_IMAGE_UBUNTU}} \
-  ${BASE_IMAGE_NGINX:+--build-arg FROM_NGINX=${BASE_IMAGE_NGINX}} \
+  ${BASE_IMAGE_UBUNTU:+--build-arg BUILD_FROM=${BASE_IMAGE_UBUNTU}} \
+  ${BASE_IMAGE_NGINX:+--build-arg FROM=${BASE_IMAGE_NGINX}} \
   --build-arg http_proxy="${HTTP_PROXY}" \
   --build-arg https_proxy="${HTTPS_PROXY}" \
   --build-arg HTTP_PROXY="${HTTP_PROXY}" \