Dockerfile: split into two stages

This shrinks resulting image size. On my amd64 system it went from
1.01 GB to 25.1 MB.

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.15
+FROM golang:1.15 as build
 
 RUN mkdir /app
 WORKDIR /app
@@ -6,8 +6,12 @@ ARG VERSION
 
 ADD . /app/
 RUN go build -ldflags=-s -ldflags=-w -ldflags=-X=github.com/miguelmota/cointop/cointop.version=$VERSION -o main .
-RUN mv main /bin/cointop
-RUN git clone https://github.com/cointop-sh/colors ~/.config/cointop/colors
+RUN git clone https://github.com/cointop-sh/colors && rm -Rf colors/.git*
 
+FROM busybox:glibc
+RUN mkdir -p /etc/ssl
+COPY --from=build /etc/ssl/certs/ /etc/ssl/certs
+COPY --from=build /app/main /bin/cointop
+COPY --from=build /app/colors /root/.config/cointop/colors
 ENTRYPOINT cointop
 CMD []