Upgrade to latest Go version + Some Docker image improvements (note)

* Rename /site to /project
* Add ldflags
* Add go and node to the default image
* Add Dart Sass to the default image
* Build the extended version by default
* Add "npm i" install support with custom entry script override
* Adjust cache logic to speed up CGO rebuilds

Closes #12920
See #12885

scripts/docker/entrypoint.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+
+# Check if a custom hugo-docker-entrypoint.sh file exists.
+if [ -f hugo-docker-entrypoint.sh ]; then
+  # Execute the custom entrypoint file.
+  sh hugo-docker-entrypoint.sh "$@"
+  exit $?
+fi
+
+# Check if a package.json file exists.
+if [ -f package.json ]; then
+  # Check if node_modules exists.
+  if [ ! -d node_modules ]; then
+    # Install npm packages.
+    # Note that we deliberately do not use `npm ci` here, as it would fail if the package-lock.json file is not up-to-date,
+    # which would be the case if you run the container with a different OS or architecture than the one used to create the package-lock.json file.
+    npm i
+  fi
+fi
+
+exec "hugo" "$@"