Environment variable to control UID of execution in docker worker.

[carter-yagemann]

The worker is currently hardcoded to invoke docker containers with the UID 1001:

mlsploit-execution-backend/mlsploit.py

Lines 136 to 142 in b4e992d

	container_logs = client.containers.run(
	'%s:latest' % module_name, auto_remove=True,
	user=1001, stdout=True, stderr=True,
	environment=['PYTHONUNBUFFERED=1'],
	volumes={
	input_dir_docker: {'bind': '/mnt/input', 'mode': 'ro'},
	output_dir_docker: {'bind': '/mnt/output', 'mode': 'rw'}})

Some docker images may want to run internally as a different UID (e.g. root), so we should expose an environment variable in .env to allow for this.

Note, worker docker images are short-lived and self-contained, so it's of no risk to the MLsploit framework if developers want their worker image to internally run as root.