docs: add info for on-error and debug-shell values for invoke

Signed-off-by: Justin Chadwell <me@jedevc.com>

docs/guides/debugging.md

@@ -49,6 +49,47 @@ bin    etc    lib    mnt    proc   run    srv    tmp    var
 dev    home   media  opt    root   sbin   sys    usr    work
 ```
 
+#### `on-error`
+
+If you want to start a debug session when a build fails, you can use
+`--invoke=on-error` to start a debug session when the build fails.
+
+```console
+$ docker buildx build --invoke on-error .
+[+] Building 4.2s (19/19) FINISHED
+ => [internal] connecting to local controller                                                                                   0.0s
+ => [internal] load build definition from Dockerfile                                                                            0.0s
+ => => transferring dockerfile: 32B                                                                                             0.0s
+ => [internal] load .dockerignore                                                                                               0.0s
+ => => transferring context: 34B                                                                                                0.0s
+ ...
+ => ERROR [shell 10/10] RUN bad-command
+------
+ > [shell 10/10] RUN bad-command:
+#0 0.049 /bin/sh: bad-command: not found
+------
+Launching interactive container. Press Ctrl-a-c to switch to monitor console
+Interactive container was restarted with process "edmzor60nrag7rh1mbi4o9lm8". Press Ctrl-a-c to switch to the new container
+/ # 
+```
+
+This allows you to explore the state of the image when the build failed.
+
+#### `debug-shell`
+
+If you want to drop into a debug session without first starting the build, you
+can use `--invoke=debug-shell` to start a debug session.
+
+```
+$ docker buildx build --invoke debug-shell .
+[+] Building 4.2s (19/19) FINISHED
+ => [internal] connecting to local controller                                                                                   0.0s
+(buildx)
+```
+
+You can then use the commands available in [monitor mode](#monitor-mode) to
+start and observe the build.
+
 ## Monitor mode
 
 By default, when debugging, you'll be dropped into a shell in the final stage.