Add tip for inspecting failed sbuilds.

doc/tips.md

@@ -18,3 +18,23 @@ Useful if your GPG key email address differs from the changelog entry:
 ```
 DEBUILD_ARGS="-k12345678" ./build_source.py pkg/foo
 ```
+
+## sbuild
+
+### Inspect a failed build
+
+Instruct `sbuild` to not purge the build directory or chroot session:
+```
+sbuild --purge never --purge-session never [...]
+```
+
+Attach to the session and look in the `/build` directory:
+```
+vagrant@packager:~/packager$ schroot -r -c
+precise-amd64-7c376c42-0b8d-49ac-abb2-0dd659a11551
+(precise-amd64)vagrant@packager:~/packager$ ls -al /build
+total 12
+drwxrws--- 1 sbuild  sbuild 4096 May 14 16:09 .
+drwxr-xr-x 1 root    root   4096 May 14 16:09 ..
+drwxr-x--- 4 vagrant sbuild 4096 May 14 16:14 foo-E8JFbs
+```