[NO TESTS NEEDED] Vendor in containers/buildah v1.20.0

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

vendor/github.com/containers/buildah/define/isolation.go

@@ -0,0 +1,32 @@
+package define
+
+import (
+	"fmt"
+)
+
+type Isolation int
+
+const (
+	// IsolationDefault is whatever we think will work best.
+	IsolationDefault Isolation = iota
+	// IsolationOCI is a proper OCI runtime.
+	IsolationOCI
+	// IsolationChroot is a more chroot-like environment: less isolation,
+	// but with fewer requirements.
+	IsolationChroot
+	// IsolationOCIRootless is a proper OCI runtime in rootless mode.
+	IsolationOCIRootless
+)
+
+// String converts a Isolation into a string.
+func (i Isolation) String() string {
+	switch i {
+	case IsolationDefault, IsolationOCI:
+		return "oci"
+	case IsolationChroot:
+		return "chroot"
+	case IsolationOCIRootless:
+		return "rootless"
+	}
+	return fmt.Sprintf("unrecognized isolation type %d", i)
+}