ci: restore dependency-storing dockerfile

.github/workflows/set-vars.sh

@@ -0,0 +1,23 @@
+#!/bin/sh
+
+msrv=$(
+    awk '/^rust-version = "[[:digit:]]+\.[[:digit:]]+\.[[:digit:]]+"$/ {
+      print substr($NF, 2, length($NF) - 2)
+    }' Cargo.toml
+)
+if [ -z $msrv ]; then
+    printf "Could not determine Rust toolchain version\n"
+    exit 1
+fi
+
+git fetch --depth=1 origin +refs/tags/*:refs/tags/*
+if ! git diff HEAD~1 --quiet -- "$UBUNTU_DOCKERFILE" ||
+    git describe --candidates=0; then
+    should_build=true
+else
+    should_build=false
+fi
+
+echo "msrv=$msrv" >> "$GITHUB_OUTPUT"
+echo "should_build=$should_build" >> "$GITHUB_OUTPUT"
+echo "container_path=$REGISTRY/$1" >> "$GITHUB_OUTPUT"