feat: add private Docker registry authentication for base images

Add per-app registry credentials that are passed to Docker during image
builds, allowing apps to use base images from private registries.

- New registry_credentials table (migration 007)
- RegistryCredential model with full CRUD operations
- Docker client passes AuthConfigs to ImageBuild when credentials exist
- Deploy service fetches app registry credentials before builds
- Web UI section for managing registry credentials (add/edit/delete)
- Comprehensive unit tests for model and auth config builder
- README updated to list the feature

internal/docker/auth_test.go

@@ -0,0 +1,96 @@
+package docker //nolint:testpackage // tests unexported buildAuthConfigs
+
+import (
+	"testing"
+)
+
+func TestBuildAuthConfigsEmpty(t *testing.T) {
+	t.Parallel()
+
+	result := buildAuthConfigs(nil)
+	if len(result) != 0 {
+		t.Errorf("expected empty map, got %d entries", len(result))
+	}
+}
+
+func TestBuildAuthConfigsSingle(t *testing.T) {
+	t.Parallel()
+
+	auths := []RegistryAuth{
+		{
+			Registry: "registry.example.com",
+			Username: "user",
+			Password: "pass",
+		},
+	}
+
+	result := buildAuthConfigs(auths)
+
+	if len(result) != 1 {
+		t.Fatalf("expected 1 entry, got %d", len(result))
+	}
+
+	cfg, ok := result["registry.example.com"]
+	if !ok {
+		t.Fatal("expected registry.example.com key")
+	}
+
+	if cfg.Username != "user" {
+		t.Errorf("expected username 'user', got %q", cfg.Username)
+	}
+
+	if cfg.Password != "pass" {
+		t.Errorf("expected password 'pass', got %q", cfg.Password)
+	}
+
+	if cfg.ServerAddress != "registry.example.com" {
+		t.Errorf("expected server address 'registry.example.com', got %q", cfg.ServerAddress)
+	}
+}
+
+func TestBuildAuthConfigsMultiple(t *testing.T) {
+	t.Parallel()
+
+	auths := []RegistryAuth{
+		{Registry: "ghcr.io", Username: "ghuser", Password: "ghtoken"},
+		{Registry: "docker.io", Username: "dkuser", Password: "dktoken"},
+	}
+
+	result := buildAuthConfigs(auths)
+
+	if len(result) != 2 {
+		t.Fatalf("expected 2 entries, got %d", len(result))
+	}
+
+	ghcr := result["ghcr.io"]
+	if ghcr.Username != "ghuser" || ghcr.Password != "ghtoken" {
+		t.Errorf("unexpected ghcr.io config: %+v", ghcr)
+	}
+
+	dkr := result["docker.io"]
+	if dkr.Username != "dkuser" || dkr.Password != "dktoken" {
+		t.Errorf("unexpected docker.io config: %+v", dkr)
+	}
+}
+
+func TestRegistryAuthStruct(t *testing.T) {
+	t.Parallel()
+
+	auth := RegistryAuth{
+		Registry: "registry.example.com",
+		Username: "testuser",
+		Password: "testpass",
+	}
+
+	if auth.Registry != "registry.example.com" {
+		t.Errorf("expected registry 'registry.example.com', got %q", auth.Registry)
+	}
+
+	if auth.Username != "testuser" {
+		t.Errorf("expected username 'testuser', got %q", auth.Username)
+	}
+
+	if auth.Password != "testpass" {
+		t.Errorf("expected password 'testpass', got %q", auth.Password)
+	}
+}