add env var to append to /etc/default/graphs1090 (#225)

README.md

@@ -537,6 +537,7 @@ docker exec -it tar1090 /usr/local/bin/viewadsb --cpr-focus 3D3ED0
 | `GRAPHS1090_WWW_TITLE`                       | Set title for the web page (displayed in the browser title or tab bar)                                 | `graphs1090`   |
 | `GRAPHS1090_WWW_HEADER`                      | Set header text for the web page                                                                       | `Perf. Graphs` |
 | `GRAPHS1090_HIDE_SYSTEM`                     | Hide the system graphs and don't render them, don't collect system data                                | `no`           |
+| `GRAPHS1090_DEFAULT_APPEND`                  | Append to /etc/default/graphs1090, see <https://github.com/wiedehopf/graphs1090/blob/master/default>   | Unset          |
 
 ### Enabling UAT data
 

rootfs/etc/s6-overlay/startup.d/08-graphs1090-init

@@ -192,6 +192,10 @@ if [[ -n "${GRAPHS1090_WWW_HEADER}" ]]; then
     sed -i "s|WWW_HEADER=.*|WWW_HEADER=\"${GRAPHS1090_WWW_HEADER}\"|g" /etc/default/graphs1090
 fi
 
+if [[ -n "$GRAPHS1090_DEFAULT_APPEND" ]]; then
+    echo "$GRAPHS1090_DEFAULT_APPEND" >> /etc/default/graphs1090
+fi
+
 # Finally, add a link to graphs1090 to the tar1090 web page:
 TAR1090_INDEX_HTML="/usr/local/share/tar1090/html-webroot/index.html"
 if ! grep -qs -e 'Stats available <a href=' "$TAR1090_INDEX_HTML"; then