Add Gitea Actions CI and release workflows and deployment assets

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Dockerfile

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+# syntax=docker/dockerfile:1
+# Packages the binaries deploy/build.sh already built (dist/bin/linux_<arch>/hammerhead),
+# so the arm64 image needs no emulation. Build it with:  sh deploy/build.sh && docker buildx build .
+FROM --platform=$BUILDPLATFORM alpine:3.22 AS prep
+RUN apk add --no-cache ca-certificates \
+ && mkdir /data && chown 65532:65532 /data
+
+FROM scratch
+ARG TARGETARCH
+LABEL org.opencontainers.image.title="Hammerhead" \
+      org.opencontainers.image.description="Control plane for tunnelled ports and websites" \
+      org.opencontainers.image.source="https://git.veillax.com/Veillax/hammerhead" \
+      org.opencontainers.image.licenses="AGPL-3.0-only"
+COPY --from=prep /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
+COPY --from=prep --chown=65532:65532 /data /data
+COPY dist/bin/linux_${TARGETARCH}/hammerhead /hammerhead
+USER 65532:65532
+ENV HAMMERHEAD_DATA=/data
+VOLUME ["/data"]
+EXPOSE 8080
+ENTRYPOINT ["/hammerhead"]
+CMD ["-listen", ":8080"]