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/store/migrations.go

@@ -24,6 +24,7 @@ type migration struct {
 // entries — installed DBs key off the id column.
 var migrations = []migration{
 	{id: 1, name: "baseline", up: migrateBaseline},
+	{id: 2, name: "drop_images_docker", up: migrateDropImagesDocker},
 }
 
 // runMigrations ensures schema_migrations exists, then applies every
@@ -141,3 +142,13 @@ func migrateBaseline(tx *sql.Tx) error {
 	}
 	return nil
 }
+
+// migrateDropImagesDocker removes the legacy images.docker column.
+// SQLite supports ALTER TABLE ... DROP COLUMN since 3.35 (2021), and
+// banger ships against modern SQLite, so a single statement is enough.
+// Existing values are simply discarded — the field never affected
+// runtime behaviour.
+func migrateDropImagesDocker(tx *sql.Tx) error {
+	_, err := tx.Exec(`ALTER TABLE images DROP COLUMN docker;`)
+	return err
+}