Added link to multi-stage Dockerfile docs

Signed-off-by: Andre Wanlin <awanlin@spotify.com>

.changeset/poor-phones-switch.md

@@ -0,0 +1,5 @@
+---
+'@backstage/create-app': patch
+---
+
+Added link to multi-stage Dockerfile documentation as alternative option

packages/create-app/templates/default-app/packages/backend/Dockerfile

@@ -8,6 +8,9 @@
 # yarn build:backend
 #
 # Once the commands have been run, you can build the image using `yarn build-image`
+#
+# Alternatively there is also a multi-stage Dockerfile documented here:
+# https://backstage.io/docs/deployment/docker#multi-stage-build
 
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