title: pg_dump error while running rake db:migrate
body: <p>I have postgres container and a ruby container. When running docker-compose run web rake db:migrate I got an error like this</p>
  
  <pre><code>rake aborted!
  failed to execute:
  pg_dump -s -x -O -f /usr/src/app/db/structure.sql --schema=public --schema=partitioning docker_rails_dev
  
  Please check the output above for any errors and make sure that `pg_dump` is installed in your PATH and has proper permissions.
  
  /usr/local/bundle/gems/activerecord-4.2.7.1/lib/active_record/tasks/postgresql_database_tasks.rb:90:in `run_cmd'
  /usr/local/bundle/gems/activerecord-4.2.7.1/lib/active_record/tasks/postgresql_database_tasks.rb:55:in `structure_dump'
  /usr/local/bundle/gems/activerecord-4.2.7.1/lib/active_record/tasks/database_tasks.rb:183:in `structure_dump'
  /usr/local/bundle/gems/activerecord-4.2.7.1/lib/active_record/railties/databases.rake:279:in `block (3 levels) in &lt;top (required)&gt;'
  /usr/local/bundle/gems/activerecord-4.2.7.1/lib/active_record/railties/databases.rake:53:in `block (2 levels) in &lt;top (required)&gt;'
  /usr/local/bundle/gems/activerecord-4.2.7.1/lib/active_record/railties/databases.rake:45:in `block (2 levels) in &lt;top (required)&gt;'
  Tasks: TOP =&gt; db:structure:dump
  (See full trace by running task with --trace)
  </code></pre>
  
  <p>The pg_dump command is available in the postgres container but not in ruby container but I have linked them together in my <a href="http://pastebin.com/GKXuzzEQ" rel="noreferrer">docker-compose.yml</a> file. </p>