executor: Add support for inline --platform within Dockerfile

Allows end users to configure executor's `OS`, `ARCH`,`VARIANT` via inline
`--platform`.

Usage
```
FROM --platform=linux/arm64 alpine
RUN uname -a
```
While this allows executor to pull base images with custom
`OS`, `ARCH`, `VARIANT` it still allows end-users to tag images with
different format if they need to.

Signed-off-by: Aditya R <[email protected]>

imagebuildah/stage_executor.go

@@ -16,6 +16,7 @@ import (
 	"github.com/containers/buildah/define"
 	buildahdocker "github.com/containers/buildah/docker"
 	"github.com/containers/buildah/internal"
+	"github.com/containers/buildah/pkg/parse"
 	"github.com/containers/buildah/pkg/rusage"
 	"github.com/containers/buildah/util"
 	cp "github.com/containers/image/v5/copy"
@@ -595,6 +596,24 @@ func (s *StageExecutor) prepare(ctx context.Context, from string, initializeIBCo
 		}
 	}
 
+	builderSystemContext := s.executor.systemContext
+	// get platform string from stage
+	if stage.Builder.Platform != "" {
+		os, arch, variant, err := parse.Platform(stage.Builder.Platform)
+		if err != nil {
+			return nil, errors.Wrapf(err, "unable to parse platform %q", stage.Builder.Platform)
+		}
+		if arch != "" {
+			builderSystemContext.ArchitectureChoice = arch
+		}
+		if os != "" {
+			builderSystemContext.OSChoice = os
+		}
+		if variant != "" {
+			builderSystemContext.VariantChoice = variant
+		}
+	}
+
 	builderOptions := buildah.BuilderOptions{
 		Args:                  ib.Args,
 		FromImage:             from,
@@ -604,7 +623,7 @@ func (s *StageExecutor) prepare(ctx context.Context, from string, initializeIBCo
 		BlobDirectory:         s.executor.blobDirectory,
 		SignaturePolicyPath:   s.executor.signaturePolicyPath,
 		ReportWriter:          s.executor.reportWriter,
-		SystemContext:         s.executor.systemContext,
+		SystemContext:         builderSystemContext,
 		Isolation:             s.executor.isolation,
 		NamespaceOptions:      s.executor.namespaceOptions,
 		ConfigureNetwork:      s.executor.configureNetwork,

tests/bud.bats

@@ -117,6 +117,62 @@ symlink(subdir)"
   expect_output --substring $(realpath "${TESTSDIR}/bud/dockerignore3/.dockerignore")
 }
 
+# Following test must fail since we are trying to run linux/arm64 on linux/amd64
+# Issue: https://github.com/containers/buildah/issues/3712
+@test "build-with-inline-platform" {
+  # Host arch
+  run_buildah info --format '{{.host.arch}}'
+  myarch="$output"
+  otherarch="arm64"
+  for try in amd64 arm64 ppc64le s390x; do
+    if [[ "$otherarch" != "$myarch" ]]; then
+      otherarch="$otherarch"
+      break
+    fi
+  done
+  # ...create a Containerfile with --platform=linux/$otherarch
+  cat > ${TESTSDIR}/bud/platform/Dockerfile << _EOF
+FROM --platform=linux/${otherarch} alpine
+RUN uname -m
+_EOF
+
+  run_buildah '?' build --signature-policy ${TESTSDIR}/policy.json -t test ${TESTSDIR}/bud/platform
+  if [[ $status -eq 0 ]]; then
+    # Build succeeded: we must have qemu-user-static installed. Confirm that
+    # the 'uname' emits what we expect
+    expect_output --from="${lines[-1]}" "$otherarch"
+    run_buildah inspect --format '{{ .OCIv1.Architecture }}' test
+    expect_output --substring "$otherarch"
+  else
+    # Build failed: we DO NOT have qemu-user-static installed.
+    expect_output --substring "format error"
+  fi
+}
+
+# Following test must pass since we want to tag image as host arch
+# Test for use-case described here: https://github.com/containers/buildah/issues/3261
+@test "build-with-inline-platform-amd-but-tag-as-arm" {
+  # Host arch
+  run_buildah info --format '{{.host.arch}}'
+  myarch="$output"
+  targetarch="arm64"
+
+  if [[ "$targetArch" == "$myarch" ]]; then
+    targetarch="amd64"
+  fi
+
+  cat > ${TESTSDIR}/bud/platform/Dockerfile << _EOF
+FROM --platform=linux/${myarch} alpine
+RUN uname -m
+_EOF
+
+  # Tries building image where baseImage has --platform=linux/HostArch
+  run_buildah '?' build --platform linux/${targetarch} --signature-policy ${TESTSDIR}/policy.json -t test ${TESTSDIR}/bud/platform
+  run_buildah inspect --format '{{ .OCIv1.Architecture }}' test
+  # base image is pulled as HostArch but tagged as non host arch
+  expect_output --substring $targetarch
+}
+
 @test "bud-flags-order-verification" {
   run_buildah 125 build /tmp/tmpdockerfile/ -t blabla
   check_options_flag_err "-t"

tests/bud/platform/Dockerfile

@@ -0,0 +1,2 @@
+FROM --platform=linux/amd64 alpine
+RUN uname -m

tests/bud/platform/Dockerfileamd

@@ -0,0 +1,2 @@
+FROM --platform=linux/amd64 busybox
+RUN uname -m