Architecture and OS defaults & config

Always pass in an architecture and an OS, because the library's defaults are hardcoded
Default to the architecture and OS reported by the Go runtime
Allow resource to override these defaults

based loosely on concourse#36 (which no longer applies cleanly)

types.go

@@ -11,6 +11,7 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 
 	"github.com/aws/aws-sdk-go/aws"
@@ -21,6 +22,7 @@ import (
 	"github.com/aws/aws-sdk-go/service/ecr/ecriface"
 	"github.com/google/go-containerregistry/pkg/authn"
 	"github.com/google/go-containerregistry/pkg/name"
+	v1 "github.com/google/go-containerregistry/pkg/v1"
 	"github.com/google/go-containerregistry/pkg/v1/remote"
 	"github.com/google/go-containerregistry/pkg/v1/remote/transport"
 	"github.com/sirupsen/logrus"
@@ -75,6 +77,11 @@ type RegistryMirror struct {
 	BasicCredentials
 }
 
+type PlatformField struct {
+	Architecture string `json:"architecture,omitempty"`
+	OS           string `json:"os,omitempty"`
+}
+
 type Source struct {
 	Repository string `json:"repository"`
 
@@ -96,6 +103,8 @@ type Source struct {
 
 	DomainCerts []string `json:"ca_certs,omitempty"`
 
+	RawPlatform *PlatformField `json:"platform,omitempty"`
+
 	Debug bool `json:"debug,omitempty"`
 }
 
@@ -183,7 +192,29 @@ func (source Source) AuthOptions(repo name.Repository, scopeActions []string) ([
 		return nil, fmt.Errorf("initialize transport: %w", err)
 	}
 
-	return []remote.Option{remote.WithAuth(auth), remote.WithTransport(rt)}, nil
+	plat := source.Platform()
+	v1plat := v1.Platform{
+		Architecture: plat.Architecture,
+		OS:           plat.OS,
+	}
+
+	return []remote.Option{remote.WithAuth(auth), remote.WithTransport(rt), remote.WithPlatform(v1plat)}, nil
+}
+
+func (source *Source) Platform() PlatformField {
+	DefaultArchitecture := runtime.GOARCH
+	DefaultOS := runtime.GOOS
+	p := source.RawPlatform
+	if p == nil {
+		p = &PlatformField{}
+	}
+	if p.Architecture == "" {
+		p.Architecture = DefaultArchitecture
+	}
+	if p.OS == "" {
+		p.OS = DefaultOS
+	}
+	return *p
 }
 
 func (source Source) NewRepository() (name.Repository, error) {