Add cache to container build (#35697)

add mount cache directives to container builds, which speeds up local
builds bypassing node and go package download entirely on second build
and caching go compilation.
drop job level split on regular/rootless, which allows to reuse the
previously made stage for rootless, skipping duplicate builds in CI.

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

.dockerignore

@@ -74,6 +74,9 @@ cpu.out
 /VERSION
 /.air
 /.go-licenses
+/Dockerfile
+/Dockerfile.rootless
+/.venv
 
 # Files and folders that were previously generated
 /public/assets/img/webpack