I have progressed
It is now recognised when I do that:
cd /root/mailcow-dockerized
docker-compose exec rspamd-mailcow rspamadm mime modify -t footer.txt -H footer.html
NB, footer.txt and footer.html are currently located in /root/mailcow-dockerized
But, the error I have now is
Usage: rspamadm mime modify [-h] [-a <header=value>] [-r <header>]
[-R <header=pattern>] [-t <file>] [-H <file>]
<file> [<file>] …
Error: missing argument ‘file’
ERROR: 1