Massively add HCL source tag in docs Markdown files

Signed-off-by: Max Riveiro <kavu13@gmail.com>

d/registry_image.html.markdown

@@ -16,7 +16,7 @@ to date on the latest available version of the tag.
 
 ## Example Usage
 
-```
+```hcl
 data "docker_registry_image" "ubuntu" {
   name = "ubuntu:precise"
 }

index.html.markdown

@@ -18,7 +18,7 @@ Use the navigation to the left to read about the available resources.
 
 ## Example Usage
 
-```
+```hcl
 # Configure the Docker provider
 provider "docker" {
   host = "tcp://127.0.0.1:2376/"

r/container.html.markdown

@@ -12,7 +12,7 @@ Manages the lifecycle of a Docker container.
 
 ## Example Usage
 
-```
+```hcl
 # Start a container
 resource "docker_container" "ubuntu" {
   name  = "foo"
@@ -93,7 +93,7 @@ The following arguments are supported:
 
 Example:
 
-```
+```hcl
 resource "docker_container" "ubuntu" {
   name = "foo"
   image = "${docker_image.ubuntu.latest}"

r/image.html.markdown

@@ -18,7 +18,7 @@ data source to update the `pull_triggers` field.
 
 ## Example Usage
 
-```
+```hcl
 # Find the latest Ubuntu precise image.
 resource "docker_image" "ubuntu" {
   name = "ubuntu:precise"
@@ -30,7 +30,7 @@ resource "docker_image" "ubuntu" {
 
 ### Dynamic image
 
-```
+```hcl
 data "docker_registry_image" "ubuntu" {
   name = "ubuntu:precise"
 }

r/network.html.markdown

@@ -14,7 +14,7 @@ to create virtual networks within the docker environment.
 
 ## Example Usage
 
-```
+```hcl
 # Find the latest Ubuntu precise image.
 resource "docker_network" "private_network" {
   name = "my_network"

r/volume.html.markdown

@@ -14,7 +14,7 @@ to prepare volumes that can be shared across containers.
 
 ## Example Usage
 
-```
+```hcl
 # Creates a docker volume "shared_volume".
 resource "docker_volume" "shared_volume" {
   name = "shared_volume"