If I'm using Alpine 3.8, how can I add a specific package from the Alpine Edge repository? Is this even supported? There is no equivalent of backports, from what I can see.
I want to add the new version of this: https://pkgs.alpinelinux.org/package/edge/community/armhf/librdkafka
And not the older version in the 3.8 repo: https://pkgs.alpinelinux.org/package/v3.8/community/s390x/librdkafka
You could specify the exact repo to apk, using the --repository parameter.
In your case:
apk add librdkafka --repository=https://dl-cdn.alpinelinux.org/alpine/edge/community
Note that repositories listed in /etc/apk/repositories take precedence over the ones specified in command line with the --repository option.