Bugfix SMTP Dockerfile for fpm-alpine

- Typo & forgot ENV variable

.examples/dockerfiles/smtp/fpm-alpine/Dockerfile

@@ -1,5 +1,8 @@
 FROM friendica:develop-fpm-alpine
 
+# at least you HAVE to set one SMTP_HOST (normally something like mail.example.org)
+ENV SMTP_HOST mail
+
 RUN set -ex; \
     \
     apk add --no-cache \
@@ -8,7 +11,7 @@ RUN set -ex; \
     # disable the current mailhub
     sed -i "s|mailhub=|#mailhub= |g" /etc/ssmtp/ssmtp.conf; \
     # enable the new mailhub
-    echo "mailhub=${:-localhost}" >> /etc/ssmtp/ssmtp.conf;SMTP_HOST
+    echo "mailhub=${SMTP_HOST:-localhost}" >> /etc/ssmtp/ssmtp.conf;
 
 # simple = using an smtp without any credentials (mostly in local networks)
 # custom = you need to set host, port, auth_options, authinfo (e.g. for GMX support)