peg docker jekyll/builder at v3 tag

[meowsbits]

Builds with docker's jekyll/builder:latest are failing because dependency hell.

See failing build at one of the latest PRs: https://github.com/ethereumclassic/ECIPs/runs/7325591137?check_suite_focus=true (https://github.com/ethereumclassic/ECIPs/pull/489)

Proof that this actually fixes it:

• https://github.com/meowsbits/ECIPs/tree/fix/jekyll
• https://github.com/meowsbits/ECIPs/actions/runs/2678686409