images: remove the docker field

The 'docker' bit on model.Image was unused at runtime — every code
path that branched on it had been removed earlier, leaving only the
field, the SQL column, the --docker flag, and the
#feature:docker sentinel that BuildMetadataPackages emitted into a
hash file. None of those have callers anymore.

Strip the field from the model, the API params, the SQLite column,
the CLI flag, and BuildMetadataPackages's signature. Add migration
2 (drop_images_docker) so existing installs lose the column on next
daemon start. ALTER TABLE ... DROP COLUMN is fine: SQLite has
supported it since 3.35 (2021).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/cli/cli_test.go

@@ -309,7 +309,7 @@ func TestImageRegisterFlagsExist(t *testing.T) {
 	if err != nil {
 		t.Fatalf("find register: %v", err)
 	}
-	for _, flagName := range []string{"name", "rootfs", "work-seed", "kernel", "initrd", "modules", "docker"} {
+	for _, flagName := range []string{"name", "rootfs", "work-seed", "kernel", "initrd", "modules"} {
 		if register.Flags().Lookup(flagName) == nil {
 			t.Fatalf("missing flag %q", flagName)
 		}

internal/cli/commands_image.go

@@ -80,7 +80,6 @@ func (d *deps) newImageRegisterCommand() *cobra.Command {
 	cmd.Flags().StringVar(&params.InitrdPath, "initrd", "", "initrd path")
 	cmd.Flags().StringVar(&params.ModulesDir, "modules", "", "modules dir")
 	cmd.Flags().StringVar(&params.KernelRef, "kernel-ref", "", "name of a cataloged kernel (see 'banger kernel list')")
-	cmd.Flags().BoolVar(&params.Docker, "docker", false, "mark image as docker-prepared")
 	_ = cmd.RegisterFlagCompletionFunc("kernel-ref", d.completeKernelNames)
 	return cmd
 }