add proxy documentation

docs/install/linux-proxy.md

+### Running the runner behind a proxy
+
+#### Assumtions
+* you've already installed docker and the gitlab runner.
+* you are using `cntlm` as a local proxy.  
+
+The last point has **2** major advantages compared to adding the proxy details everywhere directly.  
+
+* one single source where you need to change your credentials  
+* the credentials can not be accessed from the docker runners  
+
+#### Configuring cntlm
+The config file for cntlm is `/etc/cntlm.conf`  
+Enter your username, password, domain and proxy hosts there.  
+Also make sure to remove the leading # from this line:  
+`Gateway yes`  
+**This is important! If you don't do that, your docker containers won't be able to use your local CNTLM proxy!**
+After you changed your config, restart CNTLM with:  
+`service cntlm restart`
+
+#### Proxy for Docker image download  
+Refer to https://docs.docker.com/engine/admin/systemd/#http-proxy  
+
+1. Create a systemd drop-in directory for the docker service:  
+`mkdir /etc/systemd/system/docker.service.d`  
+2. Create a file called `/etc/systemd/system/docker.service.d/http-proxy.conf` that adds the `HTTP_PROXY` environment variable:  
+
+```
+[Service]
+Environment="HTTP_PROXY=http://localhost:3128/"
+Environment="HTTPS_PROXY=http://localhost:3128/"
+```  
+
+3. Flush changes:  
+`systemctl daemon-reload`  
+4. Verify that the configuration has been loaded:  
+`systemctl show --property=Environment docker`  
+You should see:  
+`Environment=HTTP_PROXY=http://localhost:3128/ HTTPS_PROXY=http://localhost:3128/`
+5. Restart Docker:  
+`systemctl restart docker`
+
+#### Adding Proxy variables to the runner config, so it get's builds assigned from the gitlab behind the proxy  
+This is basically the same as adding the proxy to docker.  
+
+1. Create a systemd drop-in directory for the gitlab-runner service:  
+`mkdir /etc/systemd/system/gitlab-runner.service.d`  
+2. Create a file called `/etc/systemd/system/gitlab-runner.service.d/http-proxy.conf` that adds the `HTTP_PROXY` environment variable:  
+
+```
+[Service]
+Environment="HTTP_PROXY=http://localhost:3128/"
+Environment="HTTPS_PROXY=http://localhost:3128/"
+```  
+
+3. Flush changes:  
+`systemctl daemon-reload`  
+4. Verify that the configuration has been loaded:  
+`systemctl show --property=Environment gitlab-runner`  
+You should see:  
+`Environment=HTTP_PROXY=http://localhost:3128/ HTTPS_PROXY=http://localhost:3128/`
+
+
+#### adding the proxy to the docker containers (for git clone and other stuff)  
+After you registered your runner, you might want to propagate your proxy settings to the docker containers.   
+To do that you need to edit the `/etc/gitlab-runner/config.toml` and add the following to the `[[runners]]` section:  
+```
+pre_clone_script = "git config --global http.proxy $HTTP_PROXY; git config --global https.proxy $HTTPS_PROXY"
+environment = ["HTTPS_PROXY=<your_host_ip_here>:3128", "HTTP_PROXY=<your_host_ip_here>:3128"]
\ No newline at end of file