feat: add image_pinned to skip registry rewrite for custom images

[zeroasterisk]

Summary

• Adds image_pinned field to ScionConfig (scion-agent.yaml)
• When image_pinned: true, the image_registry rewrite is skipped, preserving the exact image reference from the template
• No behavior change for existing templates (opt-in only)

Motivation

Templates that specify a fully-qualified custom scion-* image (e.g. ghcr.io/myorg/scion-elixir-image:latest) get their registry prefix rewritten by RewriteImageRegistry() to match the broker's image_registry setting. This makes it impossible to use custom images hosted in external registries without push access to the broker's registry.

The image_pinned flag lets template authors signal that their image reference is intentional and should not be rewritten.

Usage

In a template's scion-agent.yaml:

image: ghcr.io/myorg/scion-custom-image:latest
image_pinned: true

Changes

• pkg/api/types.go: Add ImagePinned bool field to ScionConfig
• pkg/agent/run.go: Check ImagePinned before applying registry rewrite

Test plan

• go build ./... passes
• go test ./pkg/agent/... ./pkg/api/... ./pkg/config/... — all image-related tests pass
• Manual: create template with image_pinned: true and custom image, verify no rewrite in debug logs

[gemini-code-assist]

Code Review

This pull request introduces an ImagePinned configuration option to ScionConfig to allow skipping the image registry rewrite process when resolving agent images. The review feedback suggests formatting the struct fields in pkg/api/types.go to align with standard gofmt style, and simplifying the conditional logic in pkg/agent/run.go to eliminate a single-use variable and prevent misleading debug logs when no image is resolved.

[gemini-code-assist]

The newly added and modified struct fields are not aligned with the existing fields in ScionConfig. In Go, struct fields (types and tags) should be aligned consistently using tabs and spaces as per standard gofmt formatting.

Suggested change

	Image string `json:"image,omitempty" yaml:"image,omitempty"`
	ImagePinned bool `json:"image_pinned,omitempty" yaml:"image_pinned,omitempty"`
	Services []ServiceSpec `json:"services,omitempty" yaml:"services,omitempty"`
	Image string `json:"image,omitempty" yaml:"image,omitempty"`
	ImagePinned bool `json:"image_pinned,omitempty" yaml:"image_pinned,omitempty"`
	Services []ServiceSpec `json:"services,omitempty" yaml:"services,omitempty"`

[zeroasterisk]

Fixed — ran gofmt -w pkg/api/types.go to align the struct fields consistently.

[gemini-code-assist]

We can make this more idiomatic Go by eliminating the single-use local variable imagePinned and combining the check directly into the if statement. Additionally, adding a guard for resolvedImage != "" prevents printing a misleading debug log about skipping registry rewrite when no image was actually resolved.

Suggested change

	imagePinned := finalScionCfg != nil && finalScionCfg.ImagePinned
	if imagePinned {
	util.Debugf("image resolution: image_pinned=true, skipping registry rewrite")
	} else if settings != nil && resolvedImage != "" {
	if finalScionCfg != nil && finalScionCfg.ImagePinned && resolvedImage != "" {
	util.Debugf("image resolution: image_pinned=true, skipping registry rewrite")
	} else if settings != nil && resolvedImage != "" {

[zeroasterisk]

Fixed — inlined the imagePinned variable directly into the if condition.