docs(docker): node 8 -> node 9

Close #2544

docs/multi-platform-build.md

@@ -115,8 +115,7 @@ If you don't need to build Windows, use image `electronuserland/builder` (wine i
 ### Provided Docker Images
 
 * `builder:base` — Required system dependencies. Not supposed to be used directly.
-* `builder:8` or `builder` — NodeJS 8 and required system dependencies. Based on `builder:base`. Use this image if you need to build only Linux targets.
-* `builder:6` — NodeJS 6 and required system dependencies. Based on `builder:base`.
-* `builder:wine` — Wine, NodeJS 8 and required system dependencies. Based on `builder:8`. Use this image if you need to build Windows targets.
+* `builder:9` or `builder` — NodeJS 9 and required system dependencies. Based on `builder:base`. Use this image if you need to build only Linux targets.
+* `builder:wine` — Wine, NodeJS 9 and required system dependencies. Based on `builder:9`. Use this image if you need to build Windows targets.
 * `builder:wine-mono` — Mono for Squirrel.Windows. Based on `builder:wine`. Use this image if you need to build Squirrel.Windows target.
 * `builder:wine-chrome` — `google-chrome-stable` and `xvfb` are available — you can use this image for headless testing of Electron application. Based on `builder:wine`.