Add SKLearn and SparkML registries

[athewsey]

Issue #, if available:

Stop-gap for missing containers mentioned in #13, but doesn't provide the requested CLI input feature

Description of changes:

Conditionally log in to the appropriate ECR accounts for public SKLearn and SparkML images in the current AWS_DEFAULT_REGION, as listed in the SageMaker docs here

I figured it would be better to add the extra registries conditionally by region, since there are so many?

Testing done:

I patched the updated buildspec template into my pip installed version of the library and it enabled me to successfully build the container I was stuck on - but of course haven't been able to fully go and validate the registries for the many different regions!

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