CI: use scons cache mount for building simulator and docs (#29436)

* build docker image with scons cache

* use setup

* scons cache

* added the dockerfile

* add the shell

* docs use cache

* dont pull lfs for docs

* simulator too

* dont pull the cache image

* fix inputs

* inputs

* booleans aren't supported

* bool not supported

* bool isnt supported

* fix

* readonly cache

* fix sim cache too

* respect readonly

* gitignore and move to correct folder

* also copy old

* move to tools/ci

* move to selfdrive/test

* just make the cache writable, it's discarded anyway

* minimize diff in this pr
old-commit-hash: 197460e061

selfdrive/test/Dockerfile.scons_cache

@@ -0,0 +1,3 @@
+FROM alpine:3
+
+COPY ./scons_cache /tmp/scons_cache