Docker: Failed to get image hash. Commit may be failed!

[Startouf]

Hi,
Thanks you for this gem, it is just incredible.
I've been trying to build my own container, and deploy inside it with your gem, but for some reason I always run into a

Docker: Failed to get image hash. Commit may be failed!
At the end of the deploy invocation

Here is some debug info with --trace

01:18 deploy:log_revision
      01 echo "Branch production (at bdf8973bf8c7abfeb111f5fd88d5f9ab313f59cb) deployed as release 20161207005548 by Startouf" >> /home/app/webapp/revisions.log
    ✔ 01 app:app@(docker image: my-image/production, container: 849094b97d35) 0.116s
** Invoke deploy_into_docker:commit (first_time)
** Execute deploy_into_docker:commit
invalid reference format
01:19 deploy_into_docker:commit
      Docker: Failed to get image hash. Commit may be failed!
cap aborted!
NoMethodError: undefined method `chomp!' for nil:NilClass
/Users/Cyril/.rvm/gems/ruby-2.3.1/gems/sshkit-backend-docker-0.1.1/lib/sshkit/backend/docker.rb:218:in `docker_commit'
/Users/Cyril/.rvm/gems/ruby-2.3.1/gems/capistrano-deploy_into_docker-0.2.0/lib/capistrano/tasks/deploy_into_docker.cap:8:in `block (3 levels) in <top (required)>'
/Users/Cyril/.rvm/gems/ruby-2.3.1/gems/sshkit-1.11.3/lib/sshkit/backends/abstract.rb:29:in `instance_exec'
/Users/Cyril/.rvm/gems/ruby-2.3.1/gems/sshkit-1.11.3/lib/sshkit/backends/abstract.rb:29:in `run'
/Users/Cyril/.rvm/gems/ruby-2.3.1/gems/sshkit-1.11.3/lib/sshkit/runners/parallel.rb:12:in `block (2 levels) in execute'
Tasks: TOP => deploy_into_docker:commit
The deploy has failed with an error: undefined method `chomp!' for nil:NilClass
** Invoke deploy:failed (first_time)

[Startouf]

Ok there were some bugs in beckend/docker.rb, most likely the command to commit wasn't joined correctly.

Here is my suggested patch, will make a PR

if image_config["User"].to_s.length > 1
            cmd << "-c \"USER #{image_config["User"]}\""
          end

          c = image_config["Cmd"]
          if c[0] == "/bin/sh" && c[1] == "-c"
            c.shift; c.shift;
          end
          unless c.empty?
            cmd << "-c \"CMD #{c.join(' ')}\""
          end
        end

        image_name = host.docker_options[:commit]
        if image_name.is_a?(Hash)
          image_name.symbolize_keys.each do |key, val|
            if key == :name
              image_name = val
            else
              [*val].each do |v|
                cmd << "--#{key.to_s.tr('_', '-')}" << v
              end
            end
          end
        end
        cmd << container
        image_name == true or
          cmd << image_name

        pid = nil
        puts "Executing #{cmd}"
        image_hash = `#{cmd.join(' ')}`.tr('sha256:', '')
        image_hash.nil? and
          output.error "Docker: Failed to get image hash. Commit may be failed!"
        image_hash.chomp!
        ret = image_name == true ? image_hash : image_name
        output.info "Docker: commit #{container} as #{ret}"