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.stcrashreceiver

@@ -0,0 +1,18 @@
+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 stcrashreceiver && go run build.go build stcrashreceiver
+
+FROM alpine
+
+EXPOSE 8080
+
+COPY --from=builder /src/stcrashreceiver /bin/stcrashreceiver
+
+ENTRYPOINT [ "/bin/stcrashreceiver" ]