Don't enable smtputf8 when running on alpine on container restart

[envy]

postfix_disable_utf8 checks if its running on alpine and if so, disables smtputf8.

However, the check is wrong. Consider this scenario:

1. alpine container is created, smtputf8_enable is set to no using -e POSTFIX_smtputf8_enable=no
2. container is restarted. Now the elif branch is taken and smtputf8 is actually enabled on alpine.
3. Due to the set environment var, it is set to no again, so no harm done.

We actually encountered this on our system:

★★★★★ POSTFIX STARTING UP (alpine) ★★★★★
‣ NOTE  System accounts: postfix=100:101, opendkim=102:103. Careful when switching distros.
‣ INFO  Not setting any timezone for the container
‣ DEBUG /tmp writable.
‣ INFO  Using plain log format for rsyslog.
‣ NOTE  Emails in the logs will not be anonymized. Set ANONYMIZE_EMAILS to enable this feature.
‣ DEBUG Reowning root: /var/spool/postfix/
‣ DEBUG Reowning root: /var/spool/postfix/pid/
‣ DEBUG Reowning postfix:postdrop /var/spool/postfix/private/
‣ DEBUG Reowning postfix:postdrop /var/spool/postfix/public/
‣ INFO  Preparing files for Postfix chroot:
        '/etc/localtime' -> '/var/spool/postfix/etc/localtime'
        '/etc/nsswitch.conf' -> '/var/spool/postfix/etc/nsswitch.conf'
        '/etc/resolv.conf' -> '/var/spool/postfix/etc/resolv.conf'
        '/etc/services' -> '/var/spool/postfix/etc/services'
        '/etc/hosts' -> '/var/spool/postfix/etc/hosts'
        '/etc/passwd' -> '/var/spool/postfix/etc/passwd'
‣ DEBUG No upgrade of hashes needed needed.
‣ DEBUG Running on non-Alpine system. Setting smtputf8_enable=yes.
...

This PR adds an additional check for non-existence of /etfc/alpine-release to the elif branch to make sure that branch is only taken in non-alpine containers.

[bokysan]

Nice catch. Thank you for the PR.