ci: support repository dispatch events

We want to automate new container builds via the penumbra repo [0],
so that deploy actions can also update the various bots like Galileo,
without human intervention. This commit doesn't implement that
automation, but allows cross-repository API requests to trigger builds.

[0] https://github.com/penumbra-zone/penumbra

.github/workflows/container.yml

@@ -8,6 +8,13 @@ on:
         description: 'Git ref (e.g. branch or tag) of Penumbra repo for building'
         default: "main"
         required: true
+  # Support triggering builds from penumbra-zone/penumbra CI.
+  repository_dispatch:
+    inputs:
+      penumbra_version:
+        description: 'Git ref (e.g. branch or tag) of Penumbra repo for building'
+        default: "main"
+        required: true
   push:
     branches:
       - main