Container killed by oom-killer during docker update

I use docker to host several web services. There are mainly composed with Apache/nginx, PHP and a MySQL database.

Currently, I have 38 containers. If I use docker stats, I see a total of ~4GiB (/ 15.63GiB) used.

free command show the following and confirm the ram used by docker containers :

              total        used        free      shared  buff/cache   available
Mem:           15Gi       4,0Gi       226Mi       447Mi        11Gi        10Gi
Swap:         7,8Gi       622Mi       7,2Gi

During the last maintenance, I noticed an available update from 20.10.6 to 20.10.7 and updated. After the update, ~50% of the containers do not restart with error Exited 137 oom-kill. The containers have the "unless-stopped" restart policy.

During the update the ram available was Ok and clearly show when containers were killed :

In a similar situation (stop/start docker), a reboot, the issue is not present.

I would like to know why these containers were killed with oom-killer ? Should I take care of "free" RAM instead of "available" RAM ? How can I avoid this situation in the future ?