docs(Dockerfile): specify default PUID and PGID to avoid confusion

- Both of these already defaulted to 1000 in the Go code

Author: qdm12

Dockerfile

@@ -220,8 +220,8 @@ ENV VPN_SERVICE_PROVIDER=pia \
     # Extras
     VERSION_INFORMATION=on \
     TZ= \
-    PUID= \
-    PGID=
+    PUID=1000 \
+    PGID=1000
 ENTRYPOINT ["/gluetun-entrypoint"]
 EXPOSE 8000/tcp 8888/tcp 8388/tcp 8388/udp
 HEALTHCHECK --interval=5s --timeout=5s --start-period=10s --retries=3 CMD /gluetun-entrypoint healthcheck