Fix build platform environment variable (#13914)

Certain tests that make use of `pytest_wrapper.py`, such as those
triggered by `task_python_integration.sh` will fail when a "PLATFORM"
environment variable is not set within the Docker container.

When using `build.sh` to both create a container, and run a command to
execute one of the tests that requires a "PLATFORM", an error will occur
due to the missing environment variable.

This patch is necessary to add support for this environment variable to
`build.sh` and prevent such errors.

docker/build.sh

@@ -25,11 +25,15 @@
 #                [--net=host] [--cache-from <IMAGE_NAME>] [--cache]
 #                [--name CONTAINER_NAME] [--context-path <CONTEXT_PATH>]
 #                [--spec DOCKER_IMAGE_SPEC]
+#                [--platform <BUILD_PLATFORM>]
 #                [<COMMAND>]
 #
 # CONTAINER_TYPE: Type of the docker container used the run the build,
 #                 e.g. "ci_cpu", "ci_gpu"
 #
+# BUILD_PLATFORM: (Optional) Type of build platform used for the build,
+#                 e.g. "arm", "cpu", "gpu". Defaults to "cpu".
+#
 # DOCKER_IMAGE_TAG: (Optional) Docker image tag to be built and used.
 #                   Defaults to 'latest', as it is the default Docker tag.
 #
@@ -119,6 +123,14 @@ if [[ "$1" == "--name" ]]; then
     shift 2
 fi
 
+PLATFORM="cpu"
+
+if [[ "$1" == "--platform" ]]; then
+    PLATFORM="$2"
+    echo "Using build platform: ${PLATFORM}"
+    shift 2
+fi
+
 if [[ ! -f "${DOCKERFILE_PATH}" ]]; then
     echo "Invalid Dockerfile path: \"${DOCKERFILE_PATH}\""
     exit 1
@@ -227,6 +239,7 @@ if [[ -n ${COMMAND} ]]; then
         -e "CI_BUILD_GID=$(id -g)" \
         -e "CI_PYTEST_ADD_OPTIONS=$CI_PYTEST_ADD_OPTIONS" \
         -e "CI_IMAGE_NAME=${DOCKER_IMAGE_NAME}" \
+        -e "PLATFORM=${PLATFORM}" \
         ${CUDA_ENV}\
         ${CI_DOCKER_EXTRA_PARAMS[@]} \
         ${DOCKER_IMG_SPEC} \