Python base image version is retrieved in the right place (#9931)

When quick-fixing Python 3.8.4 error #9820 PYTHON_BASE_IMAGE_VERSION
variable was added but it was initialized too early in Breeze and
it took the default version of Python rather than the one chosen
by --python switch. This caused the generated requirements
(locally by Breeze only) to generate wrong set of requirements
and images built locally for different python versions were
based on default Python version, not the one chosen by --python
switch.

(cherry picked from commit 7b9e8e0)

breeze

@@ -508,6 +508,7 @@ function prepare_command_files() {
     export COMPOSE_CI_FILE
     export COMPOSE_PROD_FILE
 
+    get_base_image_version
     # Base python image for the build
     export PYTHON_BASE_IMAGE=python:${PYTHON_BASE_IMAGE_VERSION}-slim-buster
     export AIRFLOW_CI_IMAGE="${DOCKERHUB_USER}/${DOCKERHUB_REPO}:${BRANCH_NAME}-python${PYTHON_MAJOR_MINOR_VERSION}-ci"

scripts/ci/libraries/_build_images.sh

@@ -311,11 +311,17 @@ function print_build_info() {
     print_info
 }
 
+function get_base_image_version() {
+    # python image version to use
+    PYTHON_BASE_IMAGE_VERSION=${PYTHON_BASE_IMAGE_VERSION:=${PYTHON_MAJOR_MINOR_VERSION}}
+}
+
 
 
 # Prepares all variables needed by the CI build. Depending on the configuration used (python version
 # DockerHub user etc. the variables are set so that other functions can use those variables.
 function prepare_ci_build() {
+    get_base_image_version
     # We use pulled docker image cache by default for CI images to  speed up the builds
     export DOCKER_CACHE=${DOCKER_CACHE:="pulled"}
     echo
@@ -591,6 +597,7 @@ Docker building ${AIRFLOW_CI_IMAGE}.
 # Prepares all variables needed by the CI build. Depending on the configuration used (python version
 # DockerHub user etc. the variables are set so that other functions can use those variables.
 function prepare_prod_build() {
+    get_base_image_version
     # We use local docker image cache by default for Production images
     export DOCKER_CACHE=${DOCKER_CACHE:="local"}
     echo

scripts/ci/libraries/_initialization.sh

@@ -21,10 +21,6 @@ function initialize_common_environment {
     # default python Major/Minor version
     PYTHON_MAJOR_MINOR_VERSION=${PYTHON_MAJOR_MINOR_VERSION:="3.6"}
 
-    # python image version to use
-    # shellcheck disable=SC2034
-    PYTHON_BASE_IMAGE_VERSION=${PYTHON_MAJOR_MINOR_VERSION}
-
     # extra flags passed to docker run for CI image
     # shellcheck disable=SC2034
     EXTRA_DOCKER_FLAGS=()