systemd timer that runs only if another service is active (running)

docker-gc.service

[Unit]
Description=Docker garbage collection
ConditionACPower=true
ConditionDirectoryNotEmpty=/var/lib/docker
Requisite=docker.service
PartOf=docker.service
After=docker.service

[Service]
Environment=GRACE_PERIOD=1h
Type=oneshot
ExecStart=/usr/bin/docker system prune   --force --filter "until=10m"
;ExecStart=/usr/bin/docker volume prune  --force
ExecStart=/usr/bin/docker image prune -a --force --filter "until=${GRACE_PERIOD}"

docker-gc.timer

[Unit]
Description=Docker garbage collection (timer)
PartOf=docker.service
After=docker.service

[Timer]
OnStartupSec=1h
OnUnitActiveSec=2h
AccuracySec=15min

[Install]
WantedBy=docker.service