Automated tagging of default images in DockerHub (apache#15625)

The image tagging now is fully automated within the build
dockerhub script including :<VERSION> and :latest tags.

(cherry picked from commit 3d227f2)

dev/README_RELEASE_AIRFLOW.md

@@ -689,13 +689,6 @@ for python_version in "3.6" "3.7" "3.8"
 )
 ```
 
-Once this succeeds you should push the "${VERSION}" image:
-
-```shell script
-docker tag apache/airflow:${VERSION}-python3.6 apache/airflow:${VERSION}
-docker push apache/airflow:${VERSION}
-```
-
 This will wipe Breeze cache and docker-context-files in order to make sure the build is "clean". It
 also performs image verification before the images are pushed.
 

scripts/ci/images/ci_build_dockerhub.sh

@@ -134,4 +134,12 @@ else
     build_images::build_prod_images
     verify_image::verify_prod_image "${AIRFLOW_PROD_IMAGE}"
     push_pull_remove_images::push_prod_images
+    if [[ ${PYTHON_MAJOR_MINOR_VERSION} == "${DEFAULT_PYTHON_MAJOR_MINOR_VERSION}" ]]; then
+        # In case of default Python version we also push ":version" and ":latest" tag
+        docker tag "apache/airflow:${INSTALL_AIRFLOW_VERSION}-python${PYTHON_MAJOR_MINOR_VERSION}" \
+            "apache/airflow:${INSTALL_AIRFLOW_VERSION}"
+        docker tag "apache/airflow:${INSTALL_AIRFLOW_VERSION}" "apache/airflow:latest"
+        docker push "apache/airflow:${INSTALL_AIRFLOW_VERSION}"
+        docker push "apache/airflow:latest"
+    fi
 fi