use docker-cp when building layers

[joeyzhao2018]

What does this PR do?

Use docker cp to extract out the output python library.
This is the same approach used for datadog-lambda-extension.

Motivation

The previous tar cf - python | tar -xf - -C $temp_dir method tend to run into various issues, such as

• tar: Unexpected EOF in archive
• truncated tar archive error

Testing Guidelines

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)