Rails and Sidekiq containers need python3

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1646755/?referrer=gitlab_plugin

Errno::ENOENT: No such file or directory - python3
  open3.rb:213:in `spawn'
    pid = spawn(*cmd, opts)
  open3.rb:213:in `popen_run'
    pid = spawn(*cmd, opts)
  open3.rb:101:in `popen3'
    popen_run(cmd, opts, [in_r, out_w, err_w], [in_w, out_r, err_r], &block)
  open3.rb:281:in `capture3'
    popen3(*cmd, opts) {|i, o, e, t|
  github/markup/command_implementation.rb:52:in `execute'
    stdout_str, stderr_str, status = Open3.capture3(*command, stdin_data: target)
...
(114 additional frame(s) were not displayed)

Errno::ENOENT: No such file or directory - python3