feat: add support for private docker registries

[tylerslaton]

Summary

This PR adds the ability to specify the username and password to use when authenticating against a registry. It does this by adding two new flags:

• --username, -u
• --password, -p

In addition, the --content-path short name has been updated to be -c (from -p as that seems more fitting for --password).

Review Notes

1. This currently only supports writing your username/password to the console. Its fine to have this as is but we should also come up with a better way of using the docker config so that users can do this more securely locally. This is likely a good follow-up.
2. There is the ability for us to add another flag that specifies what server to use but it felt superfluous as it defaults to wherever the image is stored. I'd be happy to update it if we want that.
3. I did not add tests here as the bloat for testing private registries would delay this feature. I'd prefer get this through and worry about that complex testing logic later.

Closes #1

[exdx]

For some reason the test status is hung up -- I will manually merge

[exdx]

Thanks for the PR!