Feat: Add support for host.docker.internal for SSMTP.

[steveworley]

The generated ssmtp.conf file from the entrypoint can now determine if host.docker.internal has a valid mailhog instance running.

php:/app$ cat /etc/ssmtp/ssmtp.conf
# ssmtp config (will be filled during entrypoint 50-ssmtp.sh)

# Email 'From header's can override the default domain
FromLineOverride=yes

mailhub=host.docker.internal:1025

php:/app$ nc -zv host.docker.internal 1025
host.docker.internal (192.168.65.254:1025) open

php:/app$ sendmail -v  [email protected] < /tmp/mail.txt
[<-] 220 mailhog.example ESMTP MailHog
[->] HELO 7ab411118f33
[<-] 250 Hello 7ab411118f33
[->] MAIL FROM:<user@7ab411118f33>
[<-] 250 Sender user@7ab411118f33 ok
[->] RCPT TO:<[email protected]>
[<-] 250 Recipient [email protected] ok
[->] DATA
[<-] 354 End data with <CR><LF>.<CR><LF>
[->] Received: by 7ab411118f33 (sSMTP sendmail emulation); Wed, 07 Aug 2024 02:46:20 +0000
[->] From: "user" <user@7ab411118f33>
[->] Date: Wed, 07 Aug 2024 02:46:20 +0000
[->] Subject: Test mail
[->]
[->] This is a test email.
[->] .
[<-] 250 Ok: queued as [email protected]
[->] QUIT
[<-] 221 Bye

[steveworley]

Fixes #1072

[tobybellwood]

LGTM from me too - thanks @steveworley and @stooit (and @silverham for the digging)