Add: Mail transport agent to enable email notifications in docker

This adds the `msmtp` and `msmtp-mta` packages to achieve ability
of sending email notifications from docker container. Default
`gvmd` image has no MTA but tries to send emails via local mailer
(and always fails for sure).

With msmtp user can send emails to a single preferred SMTP server.

.docker/entrypoint.sh

@@ -18,4 +18,5 @@
 
 #!/bin/bash
 
+. setup-mta
 exec gosu gvmd "$@"

.docker/prod.Dockerfile

@@ -84,6 +84,8 @@ RUN apt-get update && \
     libgpgme11 \
     libical3 \
     libpq5 \
+    msmtp \
+    msmtp-mta \
     openssh-client \
     postgresql-client-13 \
     postgresql-client-common \
@@ -107,6 +109,7 @@ COPY --from=builder /install/ /
 
 COPY .docker/start-gvmd.sh /usr/local/bin/start-gvmd
 COPY .docker/entrypoint.sh /usr/local/bin/entrypoint
+COPY .docker/setup-mta.sh /usr/local/bin/setup-mta
 
 RUN addgroup --gid 1001 --system gvmd && \
     adduser --no-create-home --shell /bin/false --disabled-password --uid 1001 --system --group gvmd

.docker/setup-mta.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+# Make any changes only when MTA_HOST has been set
+if [ -n MTA_HOST ]; then
+    echo "setting up configuration file for mail agent"
+    CONFIG="/etc/msmtprc"
+    echo "host $MTA_HOST" > $CONFIG
+    [ -n MTA_PORT ] && echo "port $MTA_PORT" >> $CONFIG
+    [ -n MTA_TLS ] && echo "tls $MTA_TLS" >> $CONFIG
+    [ -n MTA_STARTTLS ] && echo "tls_starttls $MTA_STARTTLS" >> $CONFIG
+    [ -n MTA_AUTH ] && echo "auth $MTA_AUTH" >> $CONFIG
+    [ -n MTA_USER ] && echo "user $MTA_USER" >> $CONFIG
+    [ -n MTA_FROM ] && echo "from $MTA_FROM" >> $CONFIG
+    [ -n MTA_PASSWORD ] && echo "password $MTA_PASSWORD" >> $CONFIG
+    [ -n MTA_LOGFILE ] && echo "logfile $MTA_LOGFILE" >> $CONFIG
+    chown gvmd:mail $CONFIG
+    chmod 750 $CONFIG
+fi