[doc] Update docker related information

The links to the images have been updated to use docker hub and there's
also a new paragraph about beat/beat.env.example>.
4 jobs for add_beat_env_example in 25 minutes and 27 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Build
passed #207744
docker-build
build_linux_36

00:15:24

64.0%
passed #207745
docker-build
build_linux_37

00:15:16

64.0%
passed #207742
macosx
build_macosx_36

00:25:27

64.0%
passed #207743
macosx
build_macosx_37

00:25:12

64.0%