postgrey whitelist_recipients support

[millerjason]

Context

My use case requires support for postgrey's whitelist_recipients file. Similar to whitelist_clients.local, I would like to add this support to the user config directory.

Expected Behavior

Add support for user whitelist_recipients file.

Actual Behavior

Default postgrey whitelist_recipients is provided.

Possible Fix

Add whitelist_recipients to config directory, add support for install in target/start-mailserver.sh, plus relevant tests.

    if [ -f /tmp/docker-mailserver/whitelist_recipients ]; then
            cp -f /tmp/docker-mailserver/whitelist_recipients /etc/postgrey/whitelist_recipients
    fi

Steps to Reproduce

1. Add config/whitelist_recipients
2. Start up service

Your Environment

• Amount of RAM available: 16GB
• Mailserver version used: docker-mailserver
• Docker version used: 18.03.1-ce
• Environment settings relevant to the config:
• Any relevant stack traces ("Full trace" preferred):

[johansmitsnl]

Could you provide a PR and a test for this?