Before this change, some errors could be ambiguous as they did not
distinguish a flag to be omitted, or set, but with an empty value.
For example, if a user would try to loging but with an empty username,
the error would suggest that the `--username` flag was not set (which
it was);
I don't have `MY_USERNAME` set in this shell;
printenv MY_USERNAME || echo 'variable not set'
variable not set
Now, attempting to do a non-interactive login would result in an
ambiguous error;
echo "supersecret" | docker login --password-stdin --username "$MY_USERNAME"
Must provide --username with --password-stdin
With this patch applied, the error indicates that the username was empty,
or not set;
echo "supersecret" | docker login --password-stdin --username "$MY_USERNAME"
username is empty
echo "supersecret" | docker login --password-stdin
the --password-stdin option requires --username to be set
echo "supersecret" | docker login --password-stdin --password "supersecret"
conflicting options: cannot specify both --password and --password-stdin
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
e7af1812cf