CI: build and publish every package from Woodpecker

Pushes to main run the tests. The Deploy button publishes the Windows, RPM,
deb and F-Droid packages to the same NAS directories a dev machine uses, with
the same keys, then runs update-repo.sh, so public URLs don't change.

Builds run on Debian 12, in an image defined in ci/builder, so the Linux
packages now need glibc 2.36 instead of Fedora 44's 2.43.

.woodpecker/builder.yaml

@@ -0,0 +1,23 @@
+# Build and push the image the other pipelines run in, from ci/builder.
+#
+# Trigger: open any pipeline for this repo in Woodpecker, click "Deploy", and
+# set the deploy target to `builder`.
+
+when:
+  - event: deployment
+    evaluate: 'CI_PIPELINE_DEPLOY_TARGET == "builder"'
+
+steps:
+  - name: build
+    image: woodpeckerci/plugin-docker-buildx:6.1.1
+    settings:
+      registry: registry.apps.millslan.net
+      repo: registry.apps.millslan.net/garrettmills/mtgc-builder
+      tags: latest
+      platforms: linux/amd64
+      context: ci/builder
+      dockerfile: ci/builder/Dockerfile
+      username:
+        from_secret: registry_username
+      password:
+        from_secret: registry_password