Docker login failing (at most 1 argument)

You don't have tacks in front of your options, it's some other dash like character. Try this instead:

docker login -u my-username -p my-password registry.myclient.com

While it looks similar, -u and -p are not the same as –u and –p.

This one worked for me if ci environment is in play:

echo ${MVN_PASSWORD} | docker login -u ${MVN_USER} --password-stdin ${MVN_URL}

these variables need to be set up via Settings > CI/CD > Variables (gitlabci example)