fixed #11500 Add tip about filter proxy

fixed #11500
Add tip about filter proxy to Docker installation Guides

Signed-off-by: Deshi Xiao <xiaods@gmail.com>

docs/sources/installation/debian.md

@@ -62,9 +62,16 @@ which is officially supported by Docker.
 2. Restart your system. This is necessary for Debian to use your new kernel.
 
 3. Install Docker using the get.docker.com script:
- 
+
     `curl -sSL https://get.docker.com/ | sh`
 
+>**Note**: If your company is behind a filtering proxy, you may find that the
+>`apt-key`
+>command fails for the Docker repo during installation. To work around this,
+>add the key directly using the following:
+>
+>       $ wget -qO- https://get.docker.com/gpg | sudo apt-key add -
+
 ## Giving non-root access
 
 The `docker` daemon always runs as the `root` user and the `docker`

docs/sources/installation/ubuntulinux.md

@@ -114,6 +114,12 @@ install Docker using the following:
 
 	 The system prompts you for your `sudo` password. Then, it downloads and
 	 installs Docker and its dependencies.
+>**Note**: If your company is behind a filtering proxy, you may find that the
+>`apt-key`
+>command fails for the Docker repo during installation. To work around this,
+>add the key directly using the following:
+>
+>       $ wget -qO- https://get.docker.com/gpg | sudo apt-key add -
 
 4. Verify `docker` is installed correctly.