cmd, docker: Updates for infrastructure

These are some changes to the relay pool server, upgrade server, and
crash receiver to run under Kubernetes. It's been in production for a
while.

Dockerfile.stupgrades

@@ -0,0 +1,23 @@
+ARG GOVERSION=latest
+FROM golang:$GOVERSION AS builder
+
+WORKDIR /src
+COPY . .
+
+ENV CGO_ENABLED=0
+ENV BUILD_HOST=syncthing.net
+ENV BUILD_USER=docker
+RUN rm -f stupgrades && go run build.go build stupgrades
+
+FROM alpine
+
+EXPOSE 8080
+
+COPY --from=builder /src/stupgrades /bin/stupgrades
+
+ENTRYPOINT [ \
+	"/bin/stupgrades", \
+	"-f", "/nightly.json->https://build.syncthing.net/guestAuth/repository/download/Release_Nightly/.lastSuccessful/nightly.json", \
+	"-f", "/syncthing-macos/appcast.xml->https://build.syncthing.net/guestAuth/repository/download/SyncthingMacOS_CreateAppcastXml/.lastSuccessful/appcast.xml" \
+	]
+