Build docker image using multi-stage build

This further decreases the image size to 14.6MB.

Dockerfile

@@ -1,16 +1,11 @@
-FROM alpine:3.12
-
-ENV URL=http://localhost:8080
-
+# builder image:
+FROM alpine:3.12 as build
 ARG build_deps="go git"
 ARG runtime_deps="dumb-init"
-ARG user_id=1000
 
 COPY . /go/src/github.com/elisescu/tty-proxy
-
 RUN apk update && \
     apk add -u $build_deps $runtime_deps && \
-    adduser -D -H -h / -u $user_id tty-proxy && \
     cd /go/src/github.com/elisescu/tty-proxy && \
     GOPATH=/go go get github.com/go-bindata/go-bindata/... && \
     GOPATH=/go /go/bin/go-bindata --prefix static -o gobindata.go static/* && \
@@ -19,9 +14,17 @@ RUN apk update && \
     rm -r /go && \
     apk del $build_deps
 
+# runtime image:
+FROM alpine:3.12
+ARG user_id=1000
+RUN adduser -D -H -h / -u $user_id tty-proxy
+
 EXPOSE 8080
 EXPOSE 3456
 USER tty-proxy
+ENV URL=http://localhost:8080
 
 ENTRYPOINT ["/usr/bin/dumb-init", "--"]
 CMD ["/bin/sh", "-c", "/usr/bin/tty-proxy --front-address :8080 --back-address :3456 -url $URL"]
+
+COPY --from=build /usr/bin/dumb-init /usr/bin/tty-proxy /usr/bin/