Working Drupal 8 container including SSMTP.

src/mail.ini

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+sendmail_path = "/usr/sbin/ssmtp -t -i"

src/ssmtp.conf

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+#
+# /etc/ssmtp.conf -- a config file for sSMTP sendmail.
+#
+# The person who gets all mail for userids < 1000
+# Make this empty to disable rewriting.
+root=postmaster
+# The place where the mail goes. The actual machine name is required
+# no MX records are consulted. Commonly mailhosts are named mail.domain.com
+# The example will fit if you are in domain.com and you mailhub is so named.
+mailhub=nathan.nehmer.net
+# Where will the mail seem to come from?
+rewriteDomain=nehmer.net
+# The full hostname
+hostname=docker.nehmer.net
+
+UseSTARTTLS=Yes