dockerfile: Incorporate exposed ports, add volume (syncthing#4908)

Added EXPOSE to Dockerfile. this way these ports will show up in docker GUIs like cockpit.
Added VOLUME parameter, this renders creating the folder (/var/syncthing) obsolete.

Dockerfile

@@ -10,13 +10,16 @@ RUN rm -f syncthing && go run build.go build syncthing
 
 FROM alpine
 
+EXPOSE 8384 22000 21027/udp
+
+VOLUME ["/var/syncthing"]
+
 RUN apk add --no-cache ca-certificates
 
 COPY --from=builder /go/src/github.com/syncthing/syncthing/syncthing /bin/syncthing
 
 RUN echo 'syncthing:x:1000:1000::/var/syncthing:/sbin/nologin' >> /etc/passwd \
     && echo 'syncthing:!::0:::::' >> /etc/shadow \
-    && mkdir /var/syncthing \
     && chown syncthing /var/syncthing
 
 USER syncthing