The image alpine:20201218 has a problem with name resolution broken.
$ docker run --rm --name alpine2 alpine:20201218 sh -c 'apk add curl >/dev/null; curl https://gist.github.com; nslookup gist.github.com; '
curl: (6) Could not resolve host: gist.github.com
Server: 192.168.65.1
Address: 192.168.65.1:53
Non-authoritative answer:
gist.github.com canonical name = github.com
Name: github.com
Address: 140.82.113.3
Non-authoritative answer:
gist.github.com canonical name = github.com
The image alpine:3.12 works well with name resolution:
$ docker run --rm --name alpine2 alpine:3.12.3 sh -c 'apk add curl >/dev/null; curl https://gist.github.com; '
<html><body>You are being <a href="https://gist.github.com/discover">redirected</a>.</body></html>