Changes based on feedback

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

packages/create-app/templates/default-app/.gitignore.hbs

@@ -15,7 +15,7 @@ coverage
 # Dependencies
 node_modules/
 
-# Yarn 3 files
+# Yarn files
 .pnp.*
 .yarn/*
 !.yarn/patches

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

@@ -3,7 +3,7 @@
 #
 # Before building this image, be sure to have run the following commands in the repo root:
 #
-# yarn install
+# yarn install --immutable
 # yarn tsc
 # yarn build:backend
 #
@@ -26,7 +26,8 @@ RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
 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
+    apt-get install -y --no-install-recommends libsqlite3-dev && \
+    rm -rf /var/lib/apt/lists/*
 
 # From here on we use the least-privileged `node` user to run the backend.
 USER node