Support 'explorer' and 'dev' images

Issue #, if available: #27

Description of changes: To aid with development, I am introducing different image types, controlled by the following two build arguments:

build_type: This build argument has three different possible values:
- standard: This is the standard build type. it is what customer uses.
- explorer: The 'explorer' build type is almost identical to the 'standard' build type but it doesn't include the entrypoint. This is useful for debugging purposes to run the image and look around its content without starting airflow, which might require further setup.
- explorer-root: This is similar to the 'explorer' build type, but additionally uses the root user, giving the user of this Docker image elevated permissions. The user can, thus, install packages, remove packages, or anything else.
dev: When this build argument is set to True, it will result in additional packages being installed to aid with development.

For each combination of these two build arguments, a different Docker image is generated. Thus, we are currently generating these images:

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

aws / amazon-mwaa-docker-images