Create /var/spool/postfix/etc/ and /var/spool/postfix/private/ if they don't exist.

[jroose]

🗣 Description

This change creates /var/spool/postfix/etc and /var/spool/postfix/private if they do not exist.

💭 Motivation and context

The documentation recommends using /var/spool/postfix as a volume, but if an empty directory gets mounted there, then the ./etc and ./private directories don't exist and the entrypoint fails.

🧪 Testing

I just built the container, pushed it to my registry, and ran it on my K8S cluster. It no longer threw an error.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All new and existing tests pass.

✅ Pre-merge checklist

• Finalize version.

✅ Post-merge checklist

• Create a release.

[jsf9k]

Thanks for the contribution @jroose!