[17.9] Downgrade prebuilt runner helper images to Alpine 3.18

Why was this MR needed?

This partly reverts e088bed4, which was shipped in 17.9.0 helper images.

In Alpine 3.21, Git appears to have some regression supplying when supplying a bundle that doesn't exactly match the server certificates. Previously Git would fall back to using the system store, but this no longer appears to work.

For example, suppose you use the cert bundle from google.com and attempt a clone from gitlab.com:

openssl s_client -showcerts -connect google.com:443 </dev/null | openssl x509 -outform PEM > google.pem
git config --global http.sslCAInfo google.pem
git clone https://gitlab.com/group/project

In Alpine 3.21, this fails with SSL certificate problem: self-signed certificate in certificate chain.

In Alpine 3.18 - 3.20, this works fine.

Revert to Alpine 3.18 until this issue is resolved: https://gitlab.alpinelinux.org/alpine/aports/-/issues/16980

Relates to #38644 (closed)