Fix the parsing of environment variables to catch invalid ones, such as
`-e = ` or `-e =A`, early in the stack to return meaningful error
messages. Also, instead of erroring out, set unspecified env variables
as empty (e.g., `-e FOO`) to remain compatible with Docker.
Fixes: #1663
Signed-off-by: Valentin Rothberg <vrothberg@suse.com>