Building Heroku-based application using gliderlabs/herokuish docker image... Attempting to pull a previously built image for use with --cache-from... invalid reference format invalid reference format No previously cached image found. The docker build will proceed without using a cached image invalid argument "/main:ba475ff3ea7c461584a11831f1d8f1803d5a8aa7" for "-t, --tag" flag: invalid reference format See 'docker build --help'. Cleaning up file based variables ERROR: Job failed: exit code 1