Support both build-time and run-time values for WEBAPP_URL. Improve re-build time, if just the WEBAPP_URL is changed.

scripts/replace-placeholder.sh

@@ -0,0 +1,16 @@
+FROM=$1
+TO=$2
+
+if [ "${FROM}" = "${TO}" ]; then
+    echo "Nothing to replace, the value is already set to ${TO}."
+
+    exit 0
+fi
+
+# Only peform action if $FROM and $TO are different.
+echo "Replacing all statically built instances of $FROM with $TO."
+
+find apps/web/.next/ apps/web/public -type f |
+while read file; do
+    sed -i "s|$FROM|$TO|g" "$file"
+done