fix(core): dockerExec does not return stdout

[bigkraig]

First noticed when using BundlingDockerImage.cp, which is the only place where the stdout from dockerExec is needed.

It appears that the bug was introduced in #12001

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[gitpod-io]

[aws-cdk-automation]

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[jogold]

Thanks. You are right.

The correct way to solve this is https://github.com/aws/aws-cdk/pull/12258/files#diff-fdec202251e1e1d22f9e271a2032f697659fc0374ce942b85ec8952b6fb1c425R245:

const { stdout } = dockerExec(['create', this.image], {}); // Empty options to avoid stdout redirect here

#12258 has been reverted for now but should be merged again after #13152.