build(release): publish multi-arch oidcgate Docker image per release tag

- Add 'oidcgate' build entry (linux/darwin × amd64/arm64) to goreleaser.
- Add per-OS/arch tar.gz archives for the daemon binary.
- Add dockers + docker_manifests entries publishing
  ghcr.io/lukaszraczylo/oidcgate:vX.Y.Z (release tag), :vX.Y, :vX, :latest
  as multi-arch manifests (linux/amd64 + linux/arm64).
- Add cmd/oidcgate/Dockerfile (distroless static, nonroot user).
- Sign images with cosign keyless (docker_signs).
- Preserve existing source-only Traefik plugin archive via meta:true.
- Update README to advertise the published image.

cmd/oidcgate/Dockerfile

@@ -0,0 +1,28 @@
+# syntax=docker/dockerfile:1.7
+#
+# This Dockerfile is consumed by GoReleaser. The binary is built outside
+# the Docker context (by goreleaser's Go cross-compile) and placed in the
+# build context as ./oidcgate before `docker buildx build` runs.
+#
+# To build locally without goreleaser:
+#   go build -o oidcgate ./cmd/oidcgate
+#   docker build -f cmd/oidcgate/Dockerfile -t oidcgate:dev .
+FROM gcr.io/distroless/static-debian12:nonroot
+
+ARG TARGETOS
+ARG TARGETARCH
+
+LABEL org.opencontainers.image.title="oidcgate"
+LABEL org.opencontainers.image.description="Standalone OIDC forward-auth daemon for nginx/Caddy/Traefik/HAProxy/Envoy"
+LABEL org.opencontainers.image.source="https://github.com/lukaszraczylo/traefikoidc"
+LABEL org.opencontainers.image.documentation="https://github.com/lukaszraczylo/traefikoidc/blob/main/docs/OIDCGATE.md"
+LABEL org.opencontainers.image.licenses="MIT"
+
+COPY oidcgate /usr/local/bin/oidcgate
+
+EXPOSE 8080
+
+USER nonroot:nonroot
+
+ENTRYPOINT ["/usr/local/bin/oidcgate"]
+CMD ["--config", "/etc/oidcgate/config.yaml"]