Recover --docker-base-image feature

In the #866 we introduced the `--docker-base-image` to help user to specify customized build docker image (such as Linux aarch64 build).

But we noticed that it doesn’t work after the #894 , because a local var introduced and the `docker-base-image` arguments is ignored:
https://github.com/bioconda/bioconda-utils/blame/9a85115ae306f58c8b4e65e5f92f6cbdb5b68f04/bioconda_utils/cli.py#L468

This PR try to recover it, if user specify the `--docker-base-image`, it will be used directly otherwise use the local var.

Signed-off-by: Yikun Jiang <yikunkero@gmail.com>

bioconda_utils/cli.py

@@ -465,7 +465,7 @@ def build(recipe_folder, config, packages="*", git_range=None, testonly=False,
             logger.warning(f"Using tag {image_tag} for docker image, since there is no image for a not yet release version ({VERSION}).")
         else:
             image_tag = VERSION
-        docker_base_image = f"quay.io/bioconda/bioconda-utils-build-env-cos7:{image_tag}"
+        docker_base_image = docker_base_image or f"quay.io/bioconda/bioconda-utils-build-env-cos7:{image_tag}"
         logger.info(f"Using docker image {docker_base_image} for building.")
 
         docker_builder = docker_utils.RecipeBuilder(