Trouble installing pict-rs

I'm only moderately literate with Linux so I could use some help.

I'm running my Lemmy instance on a shared Linode VPS, with Lemmy being installed via YunoHost. It appears that neither pict-rs or a mail server are part of that install from what I can tell based up on how Lemmy is running for me.

I've tried following the instructions on this page:

https://git.asonix.dog/asonix/pict-rs/

I ran into some problems with docker-command not being found but I believe I've successfully installed docker but when I run this command:

sudo apt install docker-compose

I get this error:

Pulling pictrs (asonix/pictrs:v0.4.0-beta.7)... ERROR: manifest for asonix/pictrs:v0.4.0-beta.7 not found: manifest unknown: manifest unknown

Any help appreciated. If you can Zoom with me, that would be even better. Thanks,