You can run a command inside the container to list files:
docker exec tls ls /certs
The docker cp command copies files between containers and the local filesystem.
To copy from the container called tls, use:
docker cp tls:/certs/server-cert.pem .
docker cp tls:/certs/server-key.pem .
You can also copy from the local machine into the container, but the target path needs to exist:
docker exec tls mkdir /certs-backup
docker cp server-cert.pem tls:/certs-backup/
docker cp server-key.pem tls:/certs-backup/
docker exec tls ls /certs-backup
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