Dev Docker: add php ast ext

[kraftbj]

This is needed for jp phan commands.

Proposed changes:

Other information:

• Have you written new tests for your changes, if applicable?
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Jetpack product discussion

n/a

Does this pull request change what data or activity we track or use?

Testing instructions:

• Run BUILD_LOCAL=1 jp phan before and after (may need to delete local image of the dev image if present on your system)

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[anomiex]

Looks reasonable. Let's do it.

I don't know how the main Dockerfile avoids the issues with python stuff without jumping through these hoops though.