Allow to specify custom user and group using environment variables.

* SYNCTHING_USER
* SYNCTHING_USER_UID
* SYNCTHING_GROUP
* SYNCTHING_GROUP_GID

Issue #3

Dockerfile

@@ -7,6 +7,12 @@ LABEL license="MIT"
 LABEL description="Docker image for syncthing installation. \
 Think for RaspberryPi."
 
+# User definition environment variables
+ENV SYNCTHING_USER syncthing
+ENV SYNCTHING_USER_UID 500
+ENV SYNCTHING_GROUP syncthing
+ENV SYNCTHING_GROUP_GID 500
+
 # gpg: key 00654A3E: public key "Syncthing Release Management <release@syncthing.net>" imported
 ENV SYNCTHING_GPG_KEY 37C84554E7E0A261E4F76E1ED26E6ED000654A3E
 
@@ -31,20 +37,20 @@ RUN set -x \
   && rmdir "$dir" \
   && apk del .temp-deps;
 
-RUN addgroup -S -g 500 syncthing && \
-  adduser -S -u 500 -D -G syncthing syncthing;
+RUN addgroup -S -g ${SYNCTHING_GROUP_UID} ${SYNCTHING_GROUP} && \
+  adduser -S -u ${SYNCTHING_USER_UID} -D -G ${SYNCTHING_GROUP} ${SYNCTHING_USER};
 
 RUN mkdir -p /etc/syncthing /syncedfolders
 
 COPY ./config.xml /etc/syncthing
 
-RUN chown -R syncthing:syncthing /etc/syncthing
+RUN chown -R ${SYNCTHING_USER}:${SYNCTHING_GROUP} /etc/syncthing
 
 VOLUME ["/etc/syncthing"]
 
 EXPOSE 8384 22000 21027/udp
 
-USER syncthing
+USER ${SYNCTHING_USER}
 
 ENTRYPOINT ["/usr/local/bin/syncthing"]