Say that a deploy keeps only an app's volumes when the app has none (closes #248)

An app with no volume mounts now shows, in its Volume Mounts section,
that the files it writes are lost whenever a deploy or rollback replaces
its container and that a restart keeps them. Each deploy of such an app
writes the same sentence into its log, after the webhook payload and
before the clone. The README says in one sentence that a deploy or
rollback starts a new container that keeps only the files in the app's
volume mounts.

Model: opus-5-5

README.md

@@ -16,6 +16,9 @@ repositories via webhooks from Gitea, GitHub, or GitLab.
 - Notifications via ntfy and Slack-compatible webhooks
 - Simple server-rendered UI with Tailwind CSS
 
+A deploy or rollback replaces the app's container with a new one, which keeps
+only the files the app wrote to its volume mounts.
+
 ## Non-Goals
 
 - Multi-user support