Explicitly expose docker container attributes in NewCluster model

[domcorvasce]

Proposed changes

Alters the NewCluster model to expose the configuration options for clusters custom containers.

Resolves #815 even though you can already specify such config options because of NewCluster being a flexible model.

Types of changes

What types of changes does your code introduce to dbx?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

[CLAassistant]

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