Support --add-host flag for finch build command

[coderbirju]

What is the problem you're trying to solve?.
A clear and concise description of the use case for this feature. Please provide an example, if possible.

Currently finch run command supports --add-host flag that allows a user to add a custom host-to-IP mapping.
This is currently not available in the build sub command.

Describe the feature you'd like
A clear and concise description of what you'd like to happen.
The --add-host flag should be supported for the finch build command, similar to docker buildx build

Additional context
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This is currently not supported in nerdctl, so we may need to contribute upstream first.

[austinvazquez]

Opened containerd/nerdctl#3786 to add support to nerdctl.