Use RUN --mount=type=cache,... in example backend dockerfile

Signed-off-by: Boris Bera <bbera@coveo.com>

packages/backend/Dockerfile

@@ -13,9 +13,10 @@ FROM node:16-bullseye-slim
 
 # Install sqlite3 dependencies. You can skip this if you don't use sqlite3 in the image,
 # in which case you should also move better-sqlite3 to "devDependencies" in package.json.
-RUN apt-get update && \
+RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
+    --mount=type=cache,target=/var/lib/apt,sharing=locked \
+    apt-get update && \
     apt-get install -y --no-install-recommends libsqlite3-dev python3 build-essential && \
-    rm -rf /var/lib/apt/lists/* && \
     yarn config set python /usr/bin/python3
 
 # From here on we use the least-privileged `node` user to run the backend.
@@ -38,7 +39,8 @@ COPY --chown=node:node yarn.lock package.json packages/backend/dist/skeleton.tar
 RUN tar xzf skeleton.tar.gz && rm skeleton.tar.gz
 
 # Note that this install is not immutable, which is one of the reasons we don't recommend Yarn 3 yet
-RUN yarn workspaces focus --all --production && rm -rf "$(yarn cache clean)"
+RUN --mount=type=cache,uid=1000,gid=1000,target=/home/node/.yarn/berry/cache,sharing=locked \
+    yarn workspaces focus --all --production
 
 # Then copy the rest of the backend bundle, along with any other files we might want.
 COPY --chown=node:node packages/backend/dist/bundle.tar.gz app-config*.yaml ./