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

internal/docker/mounts_test.go

@@ -0,0 +1,39 @@
+package docker //nolint:testpackage // tests unexported buildMounts
+
+import (
+	"testing"
+
+	"github.com/docker/docker/api/types/mount"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestBuildMountsCreatesMissingHostPath checks that every mount asks Docker
+// to create its host path if it is missing, and keeps the rest of the volume
+// as configured.
+func TestBuildMountsCreatesMissingHostPath(t *testing.T) {
+	t.Parallel()
+
+	volumes := []VolumeMount{
+		{HostPath: "/srv/app/data", ContainerPath: "/data", ReadOnly: false},
+		{HostPath: "/srv/app/config", ContainerPath: "/etc/app", ReadOnly: true},
+	}
+
+	want := []mount.Mount{
+		{
+			Type:        mount.TypeBind,
+			Source:      "/srv/app/data",
+			Target:      "/data",
+			ReadOnly:    false,
+			BindOptions: &mount.BindOptions{CreateMountpoint: true},
+		},
+		{
+			Type:        mount.TypeBind,
+			Source:      "/srv/app/config",
+			Target:      "/etc/app",
+			ReadOnly:    true,
+			BindOptions: &mount.BindOptions{CreateMountpoint: true},
+		},
+	}
+
+	assert.Equal(t, want, buildMounts(volumes))
+}