Added a Dockerfile to build the tool in a docker container

Use an alpine base image. The image size is 40MB.

Dockerfile

@@ -0,0 +1,23 @@
+FROM golang:1.14.2-alpine3.11 as builder
+
+RUN apk add build-base
+
+WORKDIR /app
+
+COPY go.mod go.sum ./
+
+RUN go mod download
+
+COPY . .
+
+RUN go build
+
+FROM alpine:3.11
+
+RUN apk add libstdc++
+
+WORKDIR /app
+
+COPY --from=builder /app/ethdo /app
+
+ENTRYPOINT ["/app/ethdo"]