travis, Dockerfile, build: docker build and multi-arch publish combo

Dockerfile

@@ -1,3 +1,8 @@
+# Support setting various labels on the final image
+ARG COMMIT=""
+ARG VERSION=""
+ARG BUILDNUM=""
+
 # Build Geth in a stock Go builder container
 FROM golang:1.16-alpine as builder
 
@@ -14,3 +19,10 @@ COPY --from=builder /go-ethereum/build/bin/geth /usr/local/bin/
 
 EXPOSE 8545 8546 30303 30303/udp
 ENTRYPOINT ["geth"]
+
+# Add some metadata labels to help programatic image consumption
+ARG COMMIT=""
+ARG VERSION=""
+ARG BUILDNUM=""
+
+LABEL commit="$COMMIT" version="$VERSION" buildnum="$BUILDNUM"