Ignore input changes to username and password

[guineveresaenger]

The registry auth credentials can rotate frequently, depending on the container registry.
A rebuild and push should not be triggered if these fields are the only inputs that changed between updates.

This PR checks the registry input fields and only notes an rpc diff response if the server property is different.

Note that any edits to the docker build context will still trigger a rebuild - this change only affects changes to the underlying values of username and password.

Fixes #484.

• Intercept registry diff and handle its fields as a first pass at better diff
• Return NONE for diffResponse if the only updates are to username or password

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[iwahbe]

The change looks good. I'm not an expert on Docker, but this feels like a place where people will have different expectations. Before merging, can we

1. Document this change in the schema.
2. Add a test that demonstrates what the intended behavior is.

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[AaronFriel]

Noticed a docs fix.

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[AaronFriel]

Tests look good, thanks for adding those.