upgrade alpine base image to v3.15.0 (see below)
https://git.alpinelinux.org/aports/commit/main/netcat-openbsd?h=3.15-stable&id=2fccbad32270c6f49fda5fa3611be40d434cd9c1
https://github.com/fphammerle/docker-borgbackup-client/commit/04610b728c8ec3c83f7618b1641aabb97b3346af