Update build to use latest go version and create alpine based image

docker/Dockerfile

@@ -1,15 +1,14 @@
-FROM golang:1.15.6
+FROM docker.io/golang:1.16 AS build
 WORKDIR /app/gohttpserver
 ADD . /app/gohttpserver
 RUN CGO_ENABLED=0 GOOS=linux go build -ldflags '-X main.VERSION=docker' -o gohttpserver
 
-FROM debian:stretch
-WORKDIR /app
+FROM docker.io/alpine:latest
 RUN mkdir -p /app/public
-RUN apt-get update && apt-get install -y ca-certificates
 VOLUME /app/public
+WORKDIR /app
 ADD assets ./assets
-COPY --from=0 /app/gohttpserver/gohttpserver .
+COPY --from=build /app/gohttpserver/gohttpserver .
 EXPOSE 8000
 ENTRYPOINT [ "/app/gohttpserver", "--root=/app/public" ]
 CMD []