update readme with redirect to upstream substrate docker instructions. remove old docker files (#422)

Co-authored-by: Sacha Lansky <sacha@parity.io>

Author: ltfschoen

.dockerignore

@@ -6,12 +6,6 @@
 
 .DS_Store
 
-# The cache for docker container dependency
-.cargo
-
-# The cache for chain data in container
-.local
-
 # direnv files
 .envrc
 .direnv