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>/blacktip),
+# so the arm64 image needs no emulation. Build it with:  sh deploy/build.sh && docker buildx build .
+#
+# Blacktip opens ports on the machine's own IP addresses, so run it with host
+# networking (see deploy/docker-compose.yml). It runs as root in the container so it
+# can bind ports below 1024 such as 80 and 443.
+FROM --platform=$BUILDPLATFORM alpine:3.22 AS prep
+RUN apk add --no-cache ca-certificates \
+ && mkdir /data
+
+FROM scratch
+ARG TARGETARCH
+LABEL org.opencontainers.image.title="Blacktip" \
+      org.opencontainers.image.description="Edge: public ports, WireGuard tunnels, logins and mail egress" \
+      org.opencontainers.image.source="https://git.veillax.com/Veillax/blacktip" \
+      org.opencontainers.image.licenses="AGPL-3.0-only"
+COPY --from=prep /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
+COPY --from=prep /data /data
+COPY dist/bin/linux_${TARGETARCH}/blacktip /blacktip
+ENV BLACKTIP_DATA=/data
+VOLUME ["/data"]
+EXPOSE 51820/udp
+ENTRYPOINT ["/blacktip"]