Fix cat command

Author: aomarks

Dockerfile

@@ -44,12 +44,13 @@ 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
+# Kaniko doesn't include ARG values in the layer cache key (see
+# https://github.com/GoogleContainerTools/kaniko/pull/1085). 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 $PLAYGROUND_SANDBOX > playground-sandbox
+RUN echo "$PLAYGROUND_SANDBOX" > playground-sandbox
 RUN npx lerna run build --scope lit-dev-content --stream
 
 # Run the web service on container startup.