Add BUILDKIT_CONTEXT_KEEP_GIT_DIR=1 to README and docker-compose.yml

Fixes #513, #516.

Change-Id: I18d67a53db51cab622b315f153a106fda153476b

README.md

@@ -77,7 +77,8 @@ You have three options:
       hostname: synapse-admin
       build:
         context: https://github.com/Awesome-Technologies/synapse-admin.git
-        # args:
+        args:
+          - BUILDKIT_CONTEXT_KEEP_GIT_DIR=1
         #   - NODE_OPTIONS="--max_old_space_size=1024"
       ports:
         - "8080:80"

docker-compose.yml

@@ -11,6 +11,7 @@ services:
     # context: https://github.com/Awesome-Technologies/synapse-admin.git
 
     # args:
+    #   - BUILDKIT_CONTEXT_KEEP_GIT_DIR=1
     # if you're building on an architecture other than amd64, make sure
     # to define a maximum ram for node. otherwise the build will fail.
     #   - NODE_OPTIONS="--max_old_space_size=1024"