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/store_test.go

@@ -179,8 +179,8 @@ func TestGetImageRejectsMalformedTimestamp(t *testing.T) {
 	_, err := store.db.ExecContext(ctx, `
 		INSERT INTO images (
 			id, name, managed, artifact_dir, rootfs_path, kernel_path, initrd_path,
-			modules_dir, build_size, docker, created_at, updated_at
-		) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
+			modules_dir, build_size, created_at, updated_at
+		) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
 		"image-bad-time",
 		"image-bad-time",
 		0,
@@ -190,7 +190,6 @@ func TestGetImageRejectsMalformedTimestamp(t *testing.T) {
 		"",
 		"",
 		"",
-		0,
 		"not-a-time",
 		"not-a-time",
 	)
@@ -398,7 +397,6 @@ func sampleImage(name string) model.Image {
 		ModulesDir:                    "/modules/" + name,
 		BuildSize:                     "8G",
 		SeededSSHPublicKeyFingerprint: "seeded-fingerprint",
-		Docker:                        true,
 		CreatedAt:                     now,
 		UpdatedAt:                     now,
 	}