streamline the container builds and only pick out the compiled binary

Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:19.10
+FROM ubuntu:19.10 as builder
 
 RUN apt-get update && apt-get install -y build-essential
 
@@ -17,6 +17,6 @@ RUN git clone https://git.wownero.com/lza_menace/wownero-explorer && \
   ~/.cargo/bin/rustup override set nightly && \
   ~/.cargo/bin/cargo build --release
 
-WORKDIR /srv/wownero-explorer
+FROM ubuntu:19.10
 
-CMD './target/release/wownero-explorer'
+COPY --from=builder /srv/wownero-explorer/target/release/wownero-explorer /bin/wownero-explorer