Remaining disk space on docker overlay filesystem

I'm new to linux an I installed armbian to an sd card and everything works fine. The SD-Card is 64GB. Then I installed docker.io, docker-compose and portainer, nothing else. When I check for disk space with lsblk: # lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT mmcblk0 179:0 0 59.5G 0 disk ├─mmcblk0p1 179:1 0 512M 0 part /boot └─mmcblk0p2 179:2 0 58.4G 0 part / mmcblk1 179:32 0 14.6G 0 disk mmcblk1boot0 179:64 0 4M 1 disk mmcblk1boot1 179:96 0 4M 1 disk zram0 254:0 0 50M 0 disk /var/log zram1 254:1 0 929.4M 0 disk [SWAP] Then with df: # df -h Filesystem Size Used Avail Use% Mounted on udev 796M 0 796M 0% /dev tmpfs 186M 8.0M 178M 5% /run /dev/mmcblk0p2 58G 2.2G 55G 4% / tmpfs 930M 0 930M 0% /dev/shm tmpfs 5.0M 4.0K 5.0M 1% /run/lock tmpfs 930M 0 930M 0% /tmp /dev/mmcblk0p1 511M 59M 453M 12% /boot /dev/zram0 49M 7.0M 38M 16% /var/log overlay 58G 2.2G 55G 4% /var/lib/docker/overlay2/b28da5a318945ac7ae1d17d26a635edb9a662c6116dea37fb4f6c13e1c76d7d2/merged tmpfs 186M 0 186M 0% /run/user/0 Why are there 2 remaining 55 GB (/dev/mmcblk0p2 and the overlay filesystem)? Does this mean that I can only use the 55 GB space on the /var/lib/docker/overlay2/.../merged folder?