Update systemd.md

Systemctl requires different escape characters than a URL does so a proxy var with special URL characters needs to have them escaped with the hex equivalent and the systemctl escapes.

A backslash in domain\user becomes domain%5Cuser but this breaks systemctl because of the % sign.

If you need to use systemctl with a url and special chars is domain%%5Cuser (Notice the DOUBLE %).

[Service]
Environment="HTTP_PROXY=http://domain%%5Cuser:complex%%23pass@proxy.example.com:8080/"

config/daemon/systemd.md

@@ -111,6 +111,13 @@ you need to add this configuration in the Docker systemd service file.
     Environment="HTTP_PROXY=http://proxy.example.com:80"
     Environment="HTTPS_PROXY=https://proxy.example.com:443"
     ```
+        
+    If you have to use special characters in the proxy value they must be both URL encoded and escape the % signs with double %%.
+    
+    ```
+    [Service]
+    Environment="HTTP_PROXY=http://domain%%5Cuser:complex%%23pass@proxy.example.com:8080/"
+    ```
      
 3.  If you have internal Docker registries that you need to contact without
     proxying you can specify them via the `NO_PROXY` environment variable.
@@ -187,6 +194,13 @@ you need to add this configuration in the Docker systemd service file.
     Environment="HTTP_PROXY=http://proxy.example.com:80"
     Environment="HTTPS_PROXY=https://proxy.example.com:443"
     ```
+    
+    If you have to use special characters in the proxy value they must be both URL encoded and escape the % signs with double %%.
+    
+    ```
+    [Service]
+    Environment="HTTP_PROXY=http://domain%%5Cuser:complex%%23pass@proxy.example.com:8080/"
+    ```
      
 3.  If you have internal Docker registries that you need to contact without
     proxying, you can specify them via the `NO_PROXY` environment variable.