README: add specific stage build for Docker (#777)

Author: beuss

README.md

@@ -392,7 +392,8 @@ follows:
  * Run build.sh to build all stages
  * Add SKIP files to the earlier successfully built stages
  * Modify the last stage
- * Rebuild just the last stage using ```sudo CLEAN=1 ./build.sh```
+ * Rebuild just the last stage using ```sudo CLEAN=1 ./build.sh``` (or, for docker builds
+   ```PRESERVE_CONTAINER=1 CONTINUE=1 CLEAN=1 ./build-docker.sh```)
  * Once you're happy with the image you can remove the SKIP_IMAGES files and
    export your image to test