fix(dockerfile) wrong executable path

- fix postfix executable path
- expose port 25 for reference
- add health check
- bump version

Dockerfile

@@ -21,6 +21,10 @@ RUN apk --no-cache add \
     bind-tools \
     && rm -f /var/cache/apk/*
 
+EXPOSE 25
+
+HEALTHCHECK --interval=30s --timeout=5s --start-period=30s --retries=3 CMD [ "/usr/sbin/postfix", "status" ]
+
 # set environment variables
 ENV TZ=Etc/UTC
 ENV LOCAL_HOSTNAME=""
@@ -38,11 +42,11 @@ COPY entrypoint.sh /usr/local/bin/entrypoint.sh
 
 # set entrypoint and default command
 ENTRYPOINT [ "/usr/local/bin/entrypoint.sh" ]
-CMD [ "/usr/local/sbin/postfix", "start-fg" ]
+CMD [ "/usr/sbin/postfix", "start-fg" ]
 
 # set parameters, vendor, version and build-date labels
 LABEL org.label-schema.docker.params="TZ=Etc/UTC, HOSTNAME=(container hostname), DOMAINNAME=(derived from hostname), LOCAL_ENCRYPTION=false|optional|true, SMARTHOST=..., SMARTHOST_PORT=587, SMARTHOST_USERNAME=..., SMARTHOST_PASSWORD=..., SMARTHOST_ENCRYPTION=optional|false|true"
 LABEL org.label-schema.vendor="Alpine 3.13, Postfix 3.5.10-r0"
-LABEL org.label-schema.version="0.5"
+LABEL org.label-schema.version="0.9"
 ARG BUILD_DATE
 LABEL org.label-schema.build-date=${BUILD_DATE}