update all images to Alpine 3.15

The [alpine-chroot-install] upgrade is needed for [this commit] where
the Alpine 3.15 signing keys are added to the known key list.

Gotcha to be aware of:  The go ssh library may have trouble connecting
to the new version of openssh-server in Alpine 3.15 unless this line is
added to the `sshd_config`:

    PubkeyAcceptedAlgorithms +ssh-rsa

See golang/go#49952 .  Noticed with [k0sctl]
and the [make build_cluster] target.

[alpine-chroot-install]: https://github.com/alpinelinux/alpine-chroot-install
[k0sctl]: https://github.com/k0sproject/k0sctl
[make build_cluster]: https://github.com/bfritz/homelab-bootstrap/blob/v0.0.3/k0s/Makefile#L52-L53
[this commit]: alpinelinux/alpine-chroot-install@d4ab61f

Makefile

@@ -1,6 +1,6 @@
 export ACI_REPO := https://github.com/alpinelinux/alpine-chroot-install.git
-export ACI_TAG := v0.13.0
-export ALPINE_VERSION := 3.14
+export ACI_TAG := v0.13.2
+export ALPINE_VERSION := 3.15
 export APORTS_REPO := https://github.com/alpinelinux/aports.git
 export BUILD_USER := imagebuilder
 export WORK_DIR := bootstrap