Have Docker create a missing volume host path (closes #235)

Docker refused to bind-mount a host path that did not exist, so an app's
first deploy failed until someone ran mkdir on the host. buildMounts now
sets BindOptions.CreateMountpoint on every bind mount, so the Docker
daemon creates a missing host path when the container starts and leaves
an existing one alone. upaas itself cannot create it: it runs in a
container and does not see the host's filesystem. The README gains a
Volume mounts section saying so. Needs Docker Engine 23.0 or later.

Model: opus-5-5

TODO.md

@@ -20,6 +20,12 @@ regress.
 
 # Completed Steps
 
+- 2026-10-01: An app's first deploy no longer fails when a volume's host path
+  does not exist yet: upaas asks Docker to create a missing host path when the
+  app's container starts and to leave an existing one alone, so nobody has to
+  create it on the host first. The README has a Volume mounts section saying so
+  (#235).
+
 - 2026-09-29: The app page is 50% wider on large screens (84rem instead of
   56rem), its build log and container log boxes are twice as tall, the build log
   sits between the webhook URL and the environment variables, and the container