Run npmp prune and bundle clean to clean up unused files (#902)

pnpm packages and bundle gems can add significant size to the Docker
image. Ensure that we clean up those unused files to ensure we don't
add extra diskspace to the final layer when bootstrapping.

templates/web.template.yml

@@ -188,7 +188,7 @@ run:
             fi
             su discourse -c 'yarn install --frozen-lockfile && yarn cache clean'
           else
-            su discourse -c 'CI=1 pnpm install --frozen-lockfile'
+            su discourse -c 'CI=1 pnpm install --frozen-lockfile && pnpm prune'
           fi
 
   - exec:
@@ -196,6 +196,7 @@ run:
       hook: bundle_exec
       cmd:
         - su discourse -c 'bundle install --jobs $(($(nproc) - 1)) --retry 3'
+        - su discourse -c 'bundle clean'
 
   - exec:
       cd: $home