Add MSC3916-compatible giphy media repo proxy

giphyproxy/Dockerfile

@@ -0,0 +1,16 @@
+FROM golang:1-alpine AS builder
+
+RUN apk add --no-cache ca-certificates
+WORKDIR /build/giphyproxy
+COPY . /build/giphyproxy
+ENV CGO_ENABLED=0
+RUN go build -o /usr/bin/giphyproxy
+
+FROM scratch
+
+COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
+COPY --from=builder /usr/bin/giphyproxy /usr/bin/giphyproxy
+
+VOLUME /data
+WORKDIR /data
+CMD ["/usr/bin/giphyproxy"]