The ssl cert & key are used on the host server to host run a website with apache on ports 80 & 443 (hence the docker port 443 is re-directed to 554), so they are valid and I've confirmed that the docker is picking them up correctly by copying them from the running docker back to the host machine & comparing them with the originals and they are exactly the same.
The output of docker ps is:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES c2c4cc713c3a dpage/pgadmin4 "/entrypoint.sh" 17 minutes ago Up 17 minutes 80/tcp, 0.0.0.0:554->443/tcp youthful_ptolemy