stage_executor: inline network add default string

Inline `RUN --network=default` should do nothing. This matches
buildkit's behaviour here: https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/reference.md#run---networkdefault

Also added to podman's document here: containers/common#1366

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

imagebuildah/stage_executor.go

@@ -635,7 +635,7 @@ func (s *StageExecutor) Run(run imagebuilder.Run, config docker.Config) error {
 		options.ConfigureNetwork = define.NetworkEnabled
 	case "none":
 		options.ConfigureNetwork = define.NetworkDisabled
-	case "":
+	case "", "default":
 		// do nothing
 	default:
 		return fmt.Errorf(`unsupported value %q for "RUN --network", must be either "host" or "none"`, run.Network)

tests/bud.bats

@@ -43,6 +43,19 @@ load helpers
   expect_output --substring "unsupported value"
 }
 
+@test "build with inline default RUN --network=default" {
+  _prefetch alpine
+  run readlink /proc/self/ns/net
+  hostns=$output
+  run_buildah build --network=host $WITH_POLICY_JSON -t source -f $BUDFILES/inline-network/Dockerfile4
+  firstns=${lines[2]}
+  assert "${hostns}" == "$firstns"
+  run_buildah build --network=private $WITH_POLICY_JSON -t source -f $BUDFILES/inline-network/Dockerfile4
+  secondns=${lines[2]}
+  assert "$secondns" != "$firstns"
+  assert "$secondns" != "$hostns"
+}
+
 
 @test "bud with ignoresymlink on default file" {
 	  cat > /tmp/private_file << _EOF

tests/bud/inline-network/Dockerfile4

@@ -0,0 +1,2 @@
+FROM alpine
+RUN --network=default readlink /proc/self/ns/net