slack-config: add apko channel
slack-config: add apko channel

apko is a free and open source software tool for building container images
using apk, the package manager most typically used in Alpine Linux.

Signed-off-by: Ariadne Conill <8f97824a610f7ff205e346cff94d8c657b8c9ff7@chainguard.dev>

kubernetes/community,kubernetes/community,kubernetes/community