[RFC] Support Bazel platform mappings

This is a proof of concept/RFC to support Bazel platforms in skaffold.

Skaffold uses github.com/opencontainers/image-spec/specs-go/v1 and
containerd strings like "linux/amd64" or "linux/arm64/v7" to represent
platforms.

Bazel's platform system uses bazel targets to define platforms
(https://bazel.build/extending/platforms). While there are common
cpu and os definitions available, each Bazel project defines its own
target platforms (!)

Currently, when skaffold invokes bazel, skaffold's platform information
is not passed to Bazel. Bazel defaults to the host platform, unless the
target skaffold is building explicitly specifies a platform.

This change allows a build-level setting for configuring Bazel and
defines a platform mapping from Skaffold platforms to Bazel platforms.
The Bazel artifact builder then passes the --platforms flag to Bazel if applicable.

This allows a Bazel + Skaffold project to flexibly target multiple platforms.

This PR is intended to start a discussion w/ the Skaffold team on whether
this should be supported and if so, how it should be implemented. If there's
interest we can discuss pushing forward from there. This doesn't include
any tests or documentation, and is not intended to be merged as-is.

pkg/skaffold/build/bazel/build.go

@@ -45,7 +45,7 @@ func (b *Builder) Build(ctx context.Context, out io.Writer, artifact *latest.Art
 
 	a := artifact.ArtifactType.BazelArtifact
 
-	tarPath, err := b.buildTar(ctx, out, artifact.Workspace, a)
+	tarPath, err := b.buildTar(ctx, out, artifact.Workspace, a, matcher)
 	if err != nil {
 		return "", err
 	}
@@ -58,7 +58,7 @@ func (b *Builder) Build(ctx context.Context, out io.Writer, artifact *latest.Art
 
 func (b *Builder) SupportedPlatforms() platform.Matcher { return platform.All }
 
-func (b *Builder) buildTar(ctx context.Context, out io.Writer, workspace string, a *latest.BazelArtifact) (string, error) {
+func (b *Builder) buildTar(ctx context.Context, out io.Writer, workspace string, a *latest.BazelArtifact, matcher platform.Matcher) (string, error) {
 	if !strings.HasSuffix(a.BuildTarget, ".tar") {
 		return "", errors.New("the bazel build target should end with .tar, see https://github.com/bazelbuild/rules_docker#using-with-docker-locally")
 	}
@@ -67,6 +67,16 @@ func (b *Builder) buildTar(ctx context.Context, out io.Writer, workspace string,
 	args = append(args, a.BuildArgs...)
 	args = append(args, a.BuildTarget)
 
+	platformMappings := a.PlatformMappings
+	for _, mapping := range platformMappings {
+		m, err := platform.Parse([]string{mapping.Platform})
+		if err == nil {
+			if matcher.Intersect(m).IsNotEmpty() {
+				args = append(args, fmt.Sprintf("--platforms=%s", mapping.BazelPlatformTarget))
+			}
+		}
+	}
+
 	if output.IsColorable(out) {
 		args = append(args, "--color=yes")
 	} else {

pkg/skaffold/schema/latest/config.go

@@ -331,6 +331,11 @@ type TaggerComponent struct {
 	Component TagPolicy `yaml:",inline" yamltags:"skipTrim"`
 }
 
+type BazelPlatformMapping struct {
+	Platform            string `yaml:"platform,omitempty"`
+	BazelPlatformTarget string `yaml:"target,omitempty"`
+}
+
 // BuildType contains the specific implementation and parameters needed
 // for the build step. Only one field should be populated.
 type BuildType struct {
@@ -1597,6 +1602,18 @@ type BazelArtifact struct {
 	// BuildArgs are additional args to pass to `bazel build`.
 	// For example: `["-flag", "--otherflag"]`.
 	BuildArgs []string `yaml:"args,omitempty"`
+
+	// PlatformMappings configure the --platforms flag for `bazel build`
+	// based on the configured skaffold target platform.
+	// For example:
+	// ```yaml
+	// platforms:
+	//   - platform: linux/amd64
+	//     target: //platforms:linux-x86_64
+	//   - platform: linux/arm64
+	//     target: //platforms:linux-arm64
+	// ```
+	PlatformMappings []BazelPlatformMapping `yaml:"platforms,omitempty"`
 }
 
 // KoArtifact builds images using [ko](https://github.com/google/ko).