fix(builder): explicitly call out $TMP_DIR on `docker cp`

https://github.com/docker/docker/commit/31d1d733037b22591e2dd2edfe6c4d2d4b8086cc
prevents a breakout in ApplyLayer, which also seems to break
`docker cp <container_id>:<src_path> .`. This change explicitly calls
out our working directory (TMP_DIR) which behaves the same as before.