Updates Dockerfile.product to have it working in the Prow CI

Dockerfile.product considers the build engine providing (a) cachito-backed cache artifacts for the dependencies and (b) a cached yarn. This commit enables builder engines not providing caching artifacts to build the Dockerfile.product image by pulling the packages from other remote sources when the cached files are not available.

Dockerfile.product

@@ -17,14 +17,29 @@ ADD . .
 
 USER 0
 
+ARG YARN_VERSION=v1.22.19
+
 # bootstrap yarn so we can install and run the other tools.
-RUN container-entrypoint npm install ./artifacts/yarn-v1.22.19.tar.gz
+RUN CACHED_YARN=./artifacts/yarn-${YARN_VERSION}.tar.gz; \
+    if [ -f ${CACHED_YARN} ]; then \
+      npm install ${CACHED_YARN}; \
+    else \
+      npm install https://github.com/yarnpkg/yarn/releases/download/${YARN_VERSION}/yarn-${YARN_VERSION}.tar.gz; \
+    fi
+
+# The REMOTE_SOURCES value is set by the build system to indicate the location of the cachito-backed artifacts cache.
+# As cachito might not be available in all environments, we need to make sure the value is set before trying to use it and
+# that the COPY layer below doesn't fail. Setting it to be the Dockerfile itself is fairly safe, as it will always be
+# available.
+ARG REMOTE_SOURCES=./Dockerfile.product
+ARG REMOTE_SOURCE_DIR=/tmp/remote-sources
 
 COPY $REMOTE_SOURCES $REMOTE_SOURCES_DIR
 
 # use dependencies provided by Cachito
-RUN cp -f $REMOTE_SOURCES_DIR/cachito-gomod-with-deps/app/registry-ca.pem . \
-  && cp -f $REMOTE_SOURCES_DIR/cachito-gomod-with-deps/app/frontend/{.npmrc,.yarnrc,yarn.lock} frontend/
+RUN test -d ${REMOTE_SOURCES}/cachito-gomod-with-deps || exit 0; \
+    cp -f $REMOTE_SOURCES_DIR/cachito-gomod-with-deps/app/registry-ca.pem . \
+ && cp -f $REMOTE_SOURCES_DIR/cachito-gomod-with-deps/app/frontend/{.npmrc,.yarnrc,yarn.lock} frontend/
 
 # prevent download of chromedriver, geckodriver, and the cypress binary as part of module installs
 ENV CHROMEDRIVER_SKIP_DOWNLOAD=true \