ci: allow the publish job to be started by hand

Also re-triggers the build, which is what this commit is for right now: the
first run built the image and then stopped at docker login, because the
registry secrets did not exist for this repository yet.

workflow_dispatch earns its place beyond that — the image content depends on
BITCOIN_VERSION, a build argument, so a new upstream release is a reason to
rebuild without a commit to point at.

.gitea/workflows/build.yaml

@@ -1,6 +1,9 @@
 name: Build docker image and push to registry.bitdeals.org
 run-name: docker build and docker push
 on:
+  # Manual trigger as well: the image also changes when upstream releases a new
+  # Core, which is a build argument here and not a commit.
+  workflow_dispatch:
   push:
     branches:
       - main