"docker run" compatibility for environment variables without values

[martin-helmich]

After #1571, mw container run will raise an Error when just passing the name of an environment variable as flag (like --env FOO). Before, this was just undefined behaviour, in which the environment variable would (probably) have been set to an empty null value.

However, docker run actually behaves differently in this case, and would extract the value of the environment variable from the users local environment, instead.

Careful; this actually works with the regular old docker run command; in which case (--env FOO), the value of the environment variable is actually just read from your shell environment.

Originally posted by @martin-helmich in #1571