Force kaniko cache invalidation on PLAYGROUND_SANDBOX value

Author: aomarks

Dockerfile

@@ -44,6 +44,12 @@ RUN npx lerna run build --scope lit-dev-api --stream && \
 # Site content
 COPY packages/lit-dev-content/ ./packages/lit-dev-content/
 ARG PLAYGROUND_SANDBOX
+# Kaniko doesn't include ARG values in the layer cache key. This is different to
+# normal Docker behavior, which would invalidate anything after the ARG
+# declaration if the value changes. So, we need to write it to the file system
+# to force a cache invalidation. Otherwise, we might re-use the most recent
+# Eleventy build output, even when the playground sandbox URL has changed.
+RUN "cat $PLAYGROUND_SANDBOX > playground-sandbox"
 RUN npx lerna run build --scope lit-dev-content --stream
 
 # Run the web service on container startup.