Update Dockerfile for arm architecture

Dockerfile.armhf

@@ -1,7 +1,13 @@
-FROM armhfbuild/alpine:3.4
+FROM golang:1.14-alpine as builder
 
-RUN apk update && \
-    apk add --no-cache ca-certificates
+WORKDIR /go/src/drone-email
+COPY . .
 
-ADD drone-email /bin/
-ENTRYPOINT ["/bin/drone-email"]
+RUN GOOS=linux GOARCH=arm GOARM=7 CGO_ENABLED=0 go build
+
+FROM alpine:3.11
+
+RUN apk add --no-cache ca-certificates
+
+COPY --from=builder /go/src/drone-email/drone-email /bin/
+ENTRYPOINT ["/bin/drone-email"]