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/daemon/imagemgr/paths.go

@@ -129,14 +129,9 @@ func StageOptionalArtifactPath(artifactDir, stagedPath, name string) string {
 }
 
 // BuildMetadataPackages returns the canonical package set recorded for a
-// managed image build. The #feature:docker sentinel is appended when
-// docker is requested.
-func BuildMetadataPackages(docker bool) []string {
-	packages := DebianBasePackages()
-	if docker {
-		packages = append(packages, "#feature:docker")
-	}
-	return packages
+// managed image build.
+func BuildMetadataPackages() []string {
+	return DebianBasePackages()
 }
 
 // WritePackagesMetadata writes the hash of packages next to rootfsPath so

internal/daemon/images.go

@@ -64,7 +64,6 @@ func (s *ImageService) RegisterImage(ctx context.Context, params api.ImageRegist
 		image.KernelPath = kernelPath
 		image.InitrdPath = initrdPath
 		image.ModulesDir = modulesDir
-		image.Docker = params.Docker
 		image.UpdatedAt = now
 	case errors.Is(lookupErr, sql.ErrNoRows):
 		id, idErr := model.NewID()
@@ -80,7 +79,6 @@ func (s *ImageService) RegisterImage(ctx context.Context, params api.ImageRegist
 			KernelPath:   kernelPath,
 			InitrdPath:   initrdPath,
 			ModulesDir:   modulesDir,
-			Docker:       params.Docker,
 			CreatedAt:    now,
 			UpdatedAt:    now,
 		}