Provide full hostname with domainname to underlying container layer

Addresses #7851

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)

daemon/container.go

@@ -1043,10 +1043,15 @@ func (container *Container) setupLinkedContainers() ([]string, error) {
 }
 
 func (container *Container) createDaemonEnvironment(linkedEnv []string) []string {
+	// if a domain name was specified, append it to the hostname (see #7851)
+	fullHostname := container.Config.Hostname
+	if container.Config.Domainname != "" {
+		fullHostname = fmt.Sprintf("%s.%s", fullHostname, container.Config.Domainname)
+	}
 	// Setup environment
 	env := []string{
 		"PATH=" + DefaultPathEnv,
-		"HOSTNAME=" + container.Config.Hostname,
+		"HOSTNAME=" + fullHostname,
 		// Note: we don't set HOME here because it'll get autoset intelligently
 		// based on the value of USER inside dockerinit, but only if it isn't
 		// set already (ie, that can be overridden by setting HOME via -e or ENV

integration-cli/docker_cli_run_test.go

@@ -821,6 +821,26 @@ func TestRunLoopbackOnlyExistsWhenNetworkingDisabled(t *testing.T) {
 	logDone("run - test loopback only exists when networking disabled")
 }
 
+// #7851 hostname outside container shows FQDN, inside only shortname
+// For testing purposes it is not required to set host's hostname directly
+// and use "--net=host" (as the original issue submitter did), as the same
+// codepath is executed with "docker run -h <hostname>".  Both were manually
+// tested, but this testcase takes the simpler path of using "run -h .."
+func TestRunFullHostnameSet(t *testing.T) {
+	cmd := exec.Command(dockerBinary, "run", "-h", "foo.bar.baz", "busybox", "hostname")
+	out, _, err := runCommandWithOutput(cmd)
+	if err != nil {
+		t.Fatal(err, out)
+	}
+
+	if actual := strings.Trim(out, "\r\n"); actual != "foo.bar.baz" {
+		t.Fatalf("expected hostname 'foo.bar.baz', received %s", actual)
+	}
+	deleteAllContainers()
+
+	logDone("run - test fully qualified hostname set with -h")
+}
+
 func TestRunPrivilegedCanMknod(t *testing.T) {
 	cmd := exec.Command(dockerBinary, "run", "--privileged", "busybox", "sh", "-c", "mknod /tmp/sda b 8 0 && echo ok")
 	out, _, err := runCommandWithOutput(cmd)