Build multi-arch (amd64+arm64) Docker images with PostgreSQL support (#793)

* Build multi-arch Docker images with PostgreSQL support

The prebuilt image now installs the `[postgresql]` extra, so the optional
PostgreSQL output backend (psycopg) works out of the box in the container
without a separate `pip install` (#792). The wheel path is resolved into a
variable before appending the extra so the shell doesn't treat
`*.whl[postgresql]` as a bracket glob.

The build workflow now sets up QEMU + Buildx and builds a multi-arch
manifest for `linux/amd64` and `linux/arm64`, so the image runs natively on
64-bit ARM hosts such as a Raspberry Pi (#789). Every compiled dependency
(psycopg[binary], lxml, maxminddb, cryptography) ships prebuilt aarch64
manylinux wheels, so the arm64 build adds no source-compilation step.

A `pull_request` trigger (scoped to the build inputs) and `workflow_dispatch`
are added so the multi-arch build can be validated on PRs and rebuilt on
demand; pushes are still gated on the release event, so neither pushes images.

Closes #789
Closes #792

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Bump version to 10.0.4 to publish the new images

The docker workflow only pushes to the registry on a `release` event, so
shipping the multi-arch + PostgreSQL-enabled image requires cutting a
release. 10.0.3 is already tagged, so bump to 10.0.4 and document the
Docker changes in the changelog.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Don't run the docker build on pull requests

The pull_request trigger (added to validate the multi-arch build) re-ran the
full ~10-minute amd64+arm64 build on every commit pushed to a docker-touching
PR, because the pull_request `paths` filter matches against the PR's entire
diff, not just the newest commit. That is wasteful once the build has been
validated.

Drop the pull_request trigger and rely on workflow_dispatch for on-demand
validation (plus the existing master-push and release triggers). Also gate the
registry login on the release event so that no non-release run authenticates
to ghcr at all — a build can only ever be pushed from a published release.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/source/usage.md

@@ -381,7 +381,11 @@ The full set of configuration options are:
   The PostgreSQL backend is an optional extra. Install it with
   `pip install parsedmarc[postgresql]` (it pulls in `psycopg`); the
   prebuilt binary wheels are not available for every platform, which is
-  why it is not a mandatory dependency.
+  why it is not a mandatory dependency. The prebuilt Docker image
+  (`ghcr.io/domainaware/parsedmarc`) already bundles this extra, so the
+  PostgreSQL backend works out of the box in the container — `psycopg`
+  ships `amd64` and `arm64` binary wheels, both of which the image
+  supports.
 
   Tables are created automatically on first run using
   `CREATE TABLE IF NOT EXISTS`, so no manual schema migration is needed