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/api/types.go

@@ -158,7 +158,6 @@ type ImageRegisterParams struct {
 	InitrdPath   string `json:"initrd_path,omitempty"`
 	ModulesDir   string `json:"modules_dir,omitempty"`
 	KernelRef    string `json:"kernel_ref,omitempty"`
-	Docker       bool   `json:"docker,omitempty"`
 }
 
 type ImagePullParams struct {