Specify the SHA256 ID instead of ubuntu:20.04

`ubuntu:20.04` is a rolling docker tag, so the image pulled from it
is not guaranteed to be the same. We should speicfy the image ID
for reproducibility.

Fixes #317

build-static.sh

@@ -14,7 +14,7 @@ set -e
   rm mold
 
 cat <<EOF | docker build -t mold-build-ubuntu20 -
-FROM ubuntu:20.04
+FROM ubuntu@sha256:cf31af331f38d1d7158470e095b132acd126a7180a54f263d386da88eb681d93
 RUN apt-get update && \
   TZ=Europe/London apt-get install -y tzdata && \
   apt-get install -y --no-install-recommends build-essential clang lld \