Add dl-4 apk repository

[jcapona]

Builds are failing because apk repos are down:

  + apk add --update --no-cache bash make g++
  fetch https://dl-cdn.alpinelinux.org/alpine/v3.15/main/x86_64/APKINDEX.tar.gz
  fetch https://dl-cdn.alpinelinux.org/alpine/v3.15/community/x86_64/APKINDEX.tar.gz
  WARNING: Ignoring https://dl-cdn.alpinelinux.org/alpine/v3.15/main: temporary error (try again later)
  WARNING: Ignoring https://dl-cdn.alpinelinux.org/alpine/v3.15/community: temporary error (try again later)
  ERROR: unable to select packages:
    bash (no such package):
      required by: world[bash]
    g++ (no such package):
      required by: world[g++]
    make (no such package):
      required by: world[make]

This PR adds another repo, dl-4, from which packages can be installed.