added docker build instructions and example config

README.md

@@ -2,3 +2,14 @@
 A turnkey GNU/Linux solution that transforms a Raspberry Pi to an Android Auto head unit.
 
 https://getcrankshaft.com/
+
+# Docker build image
+
+- Create config for pi-gen
+```bash
+cp config.example config
+```
+- Build image
+```bash
+./build-docker.sh
+```

config.example

@@ -0,0 +1,11 @@
+IMG_NAME='crankshaft-ng'
+ENABLE_SSH=0
+STAGE_LIST='stage0 stage1 stage2 stage3'
+#these are the defaults uncomment to change
+#FIRST_USER_NAME=pi
+#FIRST_USER_PASS=raspberry
+# Uncomment this if a build fails and you'd like to try again
+#CONTINUE=1
+IMG_VERSION=v1-${IMG_NAME}
+IMG_FILENAME=${IMG_VERSION}
+ZIP_FILENAME=${IMG_VERSION}