readContainerFileWithExec for links tests

Shout out to @estesp for the idea. Some use cases of
`readContainerFile` can be replaced with `docker exec $id cat $file`.
This helper method can eliminate the requirement that
host/cli should be on the same machine.

TestRunMutableNetworkFiles and TestRunResolvconfUpdater still
need to access the docker host filesystem as they modify
the file directly from there assuming cli and daemon are
on the same machine.

This fixes TestLinksUpdateOnRestart and TestLinksHostsFilesInject
for Windows CI.

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>

integration-cli/docker_cli_links_test.go

@@ -251,12 +251,12 @@ func TestLinksHostsFilesInject(t *testing.T) {
 
 	time.Sleep(1 * time.Second)
 
-	contentOne, err := readContainerFile(idOne, "hosts")
+	contentOne, err := readContainerFileWithExec(idOne, "/etc/hosts")
 	if err != nil {
 		t.Fatal(err, string(contentOne))
 	}
 
-	contentTwo, err := readContainerFile(idTwo, "hosts")
+	contentTwo, err := readContainerFileWithExec(idTwo, "/etc/hosts")
 	if err != nil {
 		t.Fatal(err, string(contentTwo))
 	}
@@ -302,7 +302,7 @@ func TestLinksUpdateOnRestart(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	content, err := readContainerFile(id, "hosts")
+	content, err := readContainerFileWithExec(id, "/etc/hosts")
 	if err != nil {
 		t.Fatal(err, string(content))
 	}
@@ -327,7 +327,7 @@ func TestLinksUpdateOnRestart(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	content, err = readContainerFile(id, "hosts")
+	content, err = readContainerFileWithExec(id, "/etc/hosts")
 	if err != nil {
 		t.Fatal(err, string(content))
 	}

integration-cli/docker_utils.go

@@ -895,6 +895,11 @@ func readContainerFile(containerId, filename string) ([]byte, error) {
 	return content, nil
 }
 
+func readContainerFileWithExec(containerId, filename string) ([]byte, error) {
+	out, _, err := runCommandWithOutput(exec.Command(dockerBinary, "exec", containerId, "cat", filename))
+	return []byte(out), err
+}
+
 func setupRegistry(t *testing.T) func() {
 	reg, err := newTestRegistryV2(t)
 	if err != nil {