Extend FAQ with clone debugging howto (#996)

docs/src/pages/faq.md

@@ -10,3 +10,42 @@ Apart from Woodpecker staying free and OpenSource forever, the growing community
 ## Why is Woodpecker a fork of Drone version 0.8?
 
 The Drone CI license was changed after the 0.8 release from Apache 2 to a proprietary license. Woodpecker is based on this latest freely available version.
+
+## How to debug clone issues:
+
+(And what to do with an error message like `fatal: could not read Username for 'https://<url>': No such device or address`)
+
+This error can have multiple causes. If you use internal repositories you might have to enable `WOODPECKER_AUTHENTICATE_PUBLIC_REPOS`:
+
+```yaml
+services:
+  woodpecker-server:
+    [...]
+    environment:
+      - [...]
+      - WOODPECKER_AUTHENTICATE_PUBLIC_REPOS=true
+```
+
+If that does not work, try to make sure the container can reach your git server. In order to do that disable git checkout and make the container "hang":
+
+```yaml
+skip_clone: true
+
+pipeline:
+  build:
+    image: debian:stable-backports
+    commands:
+      - apt update
+      - apt install -y inetutils-ping wget
+      - ping -c 4 git.example.com
+      - wget git.example.com
+      - sleep 9999999
+```
+
+Get the container id using `docker ps` and copy the id from the first column. Enter the container with: `docker exec -it 1234asdf  bash` (replace `1234asdf` with the docker id). Then try to clone the git repository with the commands from the failing pipeline:
+```bash
+$ git init
+$ git remote add origin https://git.example.com/username/repo.git 
+$ git fetch --no-tags origin +refs/heads/branch: 
+```
+(replace the url AND the branch with the correct values, use your username and password as log in values)