Change group owner of mnesia dir to 999

Relates to #234

Otherwise, the RabbitMQ process can't write the pid file into the
/var/lib/rabbitmq/mnesia/ directory on OpenShift due to permissions
denied.

Before this commit, mnesia dir was owned by user root and group root.
On OpenShift, mnesia did not have rwx bits for everyone due to stricter
security constraints:
drwxrwx---. 2 root     root       6 Aug 20 10:03 mnesia

internal/resource/statefulset.go

@@ -512,6 +512,22 @@ func (builder *StatefulSetBuilder) podTemplateSpec(annotations, labels map[strin
 						},
 					},
 				},
+				{
+					Name:  "mnesia-group-changer",
+					Image: "alpine",
+					SecurityContext: &corev1.SecurityContext{
+						RunAsUser: pointer.Int64Ptr(0),
+					},
+					VolumeMounts: []corev1.VolumeMount{
+						{
+							Name:      "persistence",
+							MountPath: "/var/lib/rabbitmq/mnesia/",
+						},
+					},
+					Command: []string{
+						"sh", "-c", "chgrp 999 /var/lib/rabbitmq/mnesia/",
+					},
+				},
 			},
 			Volumes: volumes,
 			Containers: []corev1.Container{

internal/resource/statefulset_test.go

@@ -917,15 +917,15 @@ var _ = Describe("StatefulSet", func() {
 			Expect(actualProbeCommand).To(Equal([]string{"/bin/sh", "-c", "rabbitmq-diagnostics check_port_connectivity"}))
 		})
 
-		It("templates the correct InitContainer", func() {
+		It("templates the correct InitContainers", func() {
 			stsBuilder := builder.StatefulSet()
 			Expect(stsBuilder.Update(statefulSet)).To(Succeed())
 
 			initContainers := statefulSet.Spec.Template.Spec.InitContainers
-			Expect(len(initContainers)).To(Equal(1))
+			Expect(initContainers).To(HaveLen(2))
 
-			container := extractContainer(initContainers, "copy-config")
-			Expect(container.Command).To(Equal([]string{
+			copyContainer := extractContainer(initContainers, "copy-config")
+			Expect(copyContainer.Command).To(Equal([]string{
 				"sh", "-c", "cp /tmp/rabbitmq/rabbitmq.conf /etc/rabbitmq/rabbitmq.conf && echo '' >> /etc/rabbitmq/rabbitmq.conf ; " +
 					"cp /tmp/rabbitmq/advanced.config /etc/rabbitmq/advanced.config ; " +
 					"cp /tmp/rabbitmq/rabbitmq-env.conf /etc/rabbitmq/rabbitmq-env.conf ; " +
@@ -935,8 +935,7 @@ var _ = Describe("StatefulSet", func() {
 					"cp /tmp/rabbitmq-plugins/enabled_plugins /etc/rabbitmq/enabled_plugins " +
 					"&& chown 999:999 /etc/rabbitmq/enabled_plugins",
 			}))
-
-			Expect(container.VolumeMounts).Should(ConsistOf(
+			Expect(copyContainer.VolumeMounts).To(ConsistOf(
 				corev1.VolumeMount{
 					Name:      "server-conf",
 					MountPath: "/tmp/rabbitmq/",
@@ -959,8 +958,18 @@ var _ = Describe("StatefulSet", func() {
 					MountPath: "/tmp/erlang-cookie-secret/",
 				},
 			))
+			Expect(copyContainer.Image).To(Equal("rabbitmq-image-from-cr"))
 
-			Expect(container.Image).To(Equal("rabbitmq-image-from-cr"))
+			groupChangeContainer := extractContainer(initContainers, "mnesia-group-changer")
+			Expect(groupChangeContainer.Command).To(ConsistOf("sh", "-c", "chgrp 999 /var/lib/rabbitmq/mnesia/"))
+			Expect(groupChangeContainer.VolumeMounts).To(ConsistOf(
+				corev1.VolumeMount{
+					Name:      "persistence",
+					MountPath: "/var/lib/rabbitmq/mnesia/",
+				},
+			))
+			Expect(groupChangeContainer.Image).To(Equal("alpine"))
+			Expect(groupChangeContainer.SecurityContext.RunAsUser).To(Equal(pointer.Int64Ptr(0)))
 		})
 
 		It("adds the required terminationGracePeriodSeconds", func() {