Simplify configuration

Postfix has an option to run in foreground. Use this feature to
reduce the complexity of supervisord.conf.

Deprecate some common properties and add auto-generation of DKIM keys

Remove build for linux/s390x

Add additional headers to DKIM signature

By default OpenDKIM will sign just `From`, `To`, `Sender`,
`Subject` and `Date`.

It makes sense to also sign other headers (especially `Reply-To`)
to make sure nobody fiddles with the email.

Fix for bokysan#12 where Alpine 3.9 split cyrus-sasl into multiple pa…

…ckages: https://bugs.alpinelinux.org/issues/9987

More resillient builds

It turns out that builds cannot be predictable - different accounts
were assigned by different build systems. Consequentially we install
each tool separately.

Fixed typo

Major update, better support for handling TLS connections.

This update includes a few features that make it easier to work
with relay hosts. The configuration now uses built-in (Alpine) TLS
certificates and allows the user to specify how the Postfix should
establish a TLS connection. Configure it using `RELAYHOST_TLS_LEVEL`,
if needed.

Another configuration option was added, `MESSAGE_SIZE_LIMIT`, which
enables you to reject messages exceeding certain limits. Especially
useful with relay servers, to prevent messages getting stuck in
Postfix queue.

Use the `INBOUND_DEBUGGING` parameter to enable additional postfix
logs for incoming messages when things go south.

Use `TZ` parameter to set the timezone of the container (especially
useful for logs and dates in the messages).

*Notice that `ALLOWED_SENDER_DOMAINS` is now a neccessary parameter.*
Turns out that due to Postfix anti-spam configuration there's no way
to start it properly without specifying the domains it will relay
email for.

This commit also includes a few samples of running the container:
- standalone
- in [docker-compose](https://docs.docker.com/compose/)

Further work: add a working Kubernetes / Helm chart example.