Both latest and develop tags for readarr stopped working at least a week ago ( probably even earlier ):
root@testlxc:~# docker pull linuxserver/readarr:latest
Error response from daemon: manifest for linuxserver/readarr:latest not found: manifest unknown: manifest unknown
root@testlxc:~# docker pull linuxserver/readarr:develop
develop: Pulling from linuxserver/readarr
no matching manifest for linux/amd64 in the manifest list entries