docs: Update docker installation guides for /etc/docker

Added explanation that the `/etc/docker/` directory needs to be created.

Fixes: #569.

Signed-off-by: shinebayar-g <shinebayar.gansukh@gmail.com>

install/docker/centos-docker-install.md

@@ -37,6 +37,12 @@
 
    2. Docker `daemon.json`
 
+      Create docker configuration folder.
+
+      ```
+      $ sudo mkdir -p /etc/docker
+      ```
+
       Add the following definitions to `/etc/docker/daemon.json`:
 
       ```json

install/docker/debian-docker-install.md

@@ -50,6 +50,12 @@ b. systemd (this is the default and is applied automatically if you select the
 
 c. Docker `daemon.json`
 
+    Create docker configuration folder.
+
+    ```
+    $ sudo mkdir -p /etc/docker
+    ```
+
     Add the following definitions to `/etc/docker/daemon.json`:
 
     ```json

install/docker/fedora-docker-install.md

@@ -39,6 +39,12 @@
 
    2. Docker `daemon.json`
 
+      Create docker configuration folder.
+
+      ```
+      $ sudo mkdir -p /etc/docker
+      ```
+
       Add the following definitions to `/etc/docker/daemon.json`:
 
       ```json

install/docker/opensuse-docker-install.md

@@ -37,6 +37,12 @@
 
    2. Docker `daemon.json`
 
+      Create docker configuration folder.
+
+      ```
+      $ sudo mkdir -p /etc/docker
+      ```
+
       Add the following definitions to `/etc/docker/daemon.json`:
 
       ```json

install/docker/rhel-docker-install.md

@@ -38,6 +38,12 @@
 
    2. Docker `daemon.json`
 
+      Create docker configuration folder.
+
+      ```
+      $ sudo mkdir -p /etc/docker
+      ```
+
       Add the following definitions to `/etc/docker/daemon.json`:
 
       ```json

install/docker/sles-docker-install.md

@@ -36,6 +36,12 @@
 
    2. Docker `daemon.json`
 
+      Create docker configuration folder.
+
+      ```
+      $ sudo mkdir -p /etc/docker
+      ```
+
       Add the following definitions to `/etc/docker/daemon.json`:
 
       ```json

install/docker/ubuntu-docker-install.md

@@ -41,6 +41,12 @@
 
    2. Docker `daemon.json`
 
+      Create docker configuration folder.
+
+      ```
+      $ sudo mkdir -p /etc/docker
+      ```
+
       Add the following definitions to `/etc/docker/daemon.json`:
 
       ```json