build: Use full public URL for submodules (!13)
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GitLab relative path URLs only work if the submodules are being cloned over ssh or kerberos. In the event that a clone needs to happen without credentials (e.g. building a container image) the relative path URL will fail. Given this, opt for using the full HTTPS URL.
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Include the '.git' at the end of the URL as for some reason GitLab requires this and without it the submodule clone will fail with
error: RPC failed; result=22, HTTP code = 422 fatal: The remote end hung up unexpectedly
This does not happen with GitHub, only with GitLab.