backend: remove yarn cache from built docker image

.changeset/beige-queens-crash.md

@@ -0,0 +1,13 @@
+---
+'@backstage/create-app': patch
+---
+
+Optimized the `yarn install` step in the backend `Dockerfile`.
+
+To apply these changes to an existing app, make the following changes to `packages/backend/Dockerfile`:
+
+Replace the `RUN yarn install ...` line with the following:
+
+```bash
+RUN yarn install --frozen-lockfile --production --network-timeout 600000 && rm -rf "$(yarn cache dir)"
+```