Missing Gemfile when using Docker on Heroku -

i trying out docker plugin heroku, locally start with. when run docker-compose web following error:

building web step 1 : heroku/ruby # executing 7 build triggers... step 1 : copy gemfile gemfile.lock /app/user/ error: service 'web' failed build: lstat gemfile: no such file or directory 

this docker-compose.yml:

web:   build: .   command: 'bash -c ''bundle exec puma -c config/puma.rb'''   working_dir: /app/user   environment:     port: 8080     database_url: 'postgres://postgres:@herokupostgresql:5432/postgres'   ports:     - '8080:8080'   links:     - herokupostgresql shell:   build: .   command: bash   working_dir: /app/user   environment:     port: 8080     database_url: 'postgres://postgres:@herokupostgresql:5432/postgres'   ports:     - '8080:8080'   links:     - herokupostgresql   volumes:     - '.:/app/user' herokupostgresql:   image: postgres 

why gemfile missing, importantly how should docker?