upgrade docker alpine base to 3.18

[slingamn]

See #2102, #2071, #1726. An incompatibility related to faccessat2(2) and Alpine's musl libc prevented us from upgrading the Alpine base image past 3.13. At this point:

1. Linux kernel versions that lack faccessat2(2) are still fully supported, e.g. Ubuntu 20.04 ships the 5.4 kernel, whereas 5.8 is the first version with that system call
2. However, modern Docker versions ship a workaround, e.g. Docker version 24.0.7, build afdd53b works with Alpine 3.18 under the 5.4 kernel

so let's just go ahead and upgrade it in the next Ergo release. We should changelog this as a compatibility break and include a recommended Docker version.

[slingamn]

Recommended versions here: https://about.gitlab.com/blog/2021/08/26/its-time-to-upgrade-docker-engine/

Docker 20.10.0 or higher should work.