executor: honor default ARG value while eval base name

While PR https://github.com/containers/buildah/pull/3947 added support
for evaluating `--build-args` in base image names for a builds but it
missed processing default value if any. So for scenarios where `ARG`
already has a default value in Containerfile via `ARG key=value` but
was not specified with `--build-arg key=value` the processing ignored
the default value. Following commit just adds support for that.

Makes below Containerfile functional without any external `--build-arg`
value from CLI

```Dockerfile
ARG my_env=a

FROM alpine as stage_a
RUN /bin/true

FROM stage_${my_env} as stage_b
RUN /bin/true
```

Closes: https://github.com/containers/buildah/issues/4312

Signed-off-by: Aditya R <arajan@redhat.com>

imagebuildah/executor.go

@@ -690,7 +690,12 @@ func (b *Executor) Build(ctx context.Context, stages imagebuilder.Stages) (image
 									base = child.Next.Value
 								}
 							}
+							headingArgs := argsMapToSlice(stage.Builder.HeadingArgs)
 							userArgs := argsMapToSlice(stage.Builder.Args)
+							// append heading args so if --build-arg key=value is not
+							// specified but default value is set in Containerfile
+							// via `ARG key=value` so default value can be used.
+							userArgs = append(headingArgs, userArgs...)
 							baseWithArg, err := imagebuilder.ProcessWord(base, userArgs)
 							if err != nil {
 								return "", nil, fmt.Errorf("while replacing arg variables with values for format %q: %w", base, err)

tests/bud.bats

@@ -2810,6 +2810,10 @@ _EOF
   _prefetch busybox:musl
   target=leading-args
   run_buildah build $WITH_POLICY_JSON -t ${target} --build-arg=VERSION=musl -f $BUDFILES/leading-args/Dockerfile $BUDFILES/leading-args
+
+  #Verify https://github.com/containers/buildah/issues/4312
+  # stage `FROM stage_${my_env}` must be resolved with default arg value and build should be successful.
+  run_buildah build $WITH_POLICY_JSON -t source -f $BUDFILES/multi-stage-builds/Dockerfile.arg_in_stage
 }
 
 @test "bud-with-healthcheck" {

tests/bud/multi-stage-builds/Dockerfile.arg_in_stage

@@ -0,0 +1,7 @@
+ARG my_env=a
+
+FROM alpine as stage_a
+RUN /bin/true
+
+FROM stage_${my_env} as stage_b
+RUN /bin/true