Switch to use must-gather image in build multi-stage

During transition period, when we're updating RHEL versions for oc, it's
possible that tools and cli images will be on different levels of RHEL.
Whereas cli and must-gather will match. To prevent problems in tests use
cli & must-gather in multi-stage build tests.

test/extended/builds/multistage.go

@@ -31,8 +31,8 @@ USER 1001
 FROM %[1]s as other
 COPY --from=test /usr/bin/curl /test/
 COPY --from=%[2]s /bin/echo /test/
-COPY --from=%[2]s /bin/ping /test/
-`, image.LimitedShellImage(), image.ShellImage())
+COPY --from=%[2]s /bin/wget /test/
+`, image.LimitedShellImage(), image.MustGatherImage())
 	)
 
 	g.AfterEach(func() {
@@ -124,7 +124,7 @@ COPY --from=%[2]s /bin/ping /test/
 		m, err := oc.Run("logs").Args("-f", "test", "-c", "check").Output()
 		o.Expect(err).NotTo(o.HaveOccurred())
 		o.Expect(m).To(o.ContainSubstring("echo"))
-		o.Expect(m).To(o.ContainSubstring("ping"))
+		o.Expect(m).To(o.ContainSubstring("wget"))
 		e2e.Logf("Pod logs:\n%s\n%s", string(data), string(m))
 	})
 })

test/extended/util/image/image.go

@@ -24,7 +24,9 @@ var (
 	releasePullSpecInitializationLock sync.RWMutex
 	releasePullSpecInitialized        bool
 	availablePullSpecs                = map[string]string{
-		"cli":   "image-registry.openshift-image-registry.svc:5000/openshift/cli:latest",
+		"cli":         "image-registry.openshift-image-registry.svc:5000/openshift/cli:latest",
+		"must-gather": "image-registry.openshift-image-registry.svc:5000/openshift/must-gather:latest",
+		// tools during transition period, can be on a different rhel level
 		"tools": "image-registry.openshift-image-registry.svc:5000/openshift/tools:latest",
 	}
 	imageRegistryPullSpecRegex = regexp.MustCompile(`image-registry\.openshift-image-registry\.svc:5000\/openshift\/([A-Za-z0-9._-]+)[@:A-Za-z0-9._-]*`)
@@ -180,6 +182,13 @@ func ShellImage() string {
 	return GetPullSpecForOrPanic("tools")
 }
 
+// MustGatherImage returns a docker pull spec that any pod on the cluster
+// has access to that contains bash and standard commandline tools.
+// This image has oc and must-gather scripts.
+func MustGatherImage() string {
+	return GetPullSpecForOrPanic("must-gather")
+}
+
 // LimitedShellImage returns a docker pull spec that any pod on the cluster
 // has access to that contains bash and standard commandline tools.
 // This image should be used when you only need oc and can't use the shell image.