Best practice recap (TL;DR)

Adding or cloning

• Initial add: git submodule add <url> <path>
• Initial container clone: git clone --recursive <url> [<path>]

Grabbing updates inside a submodule

• cd path/to/module
• git fetch
• git checkout -q <commit-sha1>
• cd -
• git commit -am "Updated submodule X to: blah blah"

Grabbing container updates

• git pull
• git submodule sync --recursive
• git submodule update --init --recursive

Updating a submodule inside container code

• git submodule update --remote --rebase -- path/to/module
• cd path/to/module
• Local work, testing, eventually staging
• git commit -am "Update to central submodule: blah blah"
• git push
• cd -
• git commit -am "Updated submodule X to: blah blah"

Permanently removing a submodule

• git submodule deinit path/to/module
• git rm path/to/module
• git commit -am "Removed submodule X"

Testing every elements you can find in CommonMark.