dockerfile: cp geth to /usr/local/bin

Dockerfile

@@ -4,11 +4,11 @@ ADD . /go-ethereum
 RUN \
   apk add --update git go make gcc musl-dev linux-headers && \
   (cd go-ethereum && make geth)                           && \
-  cp go-ethereum/build/bin/geth /geth                     && \
+  cp go-ethereum/build/bin/geth /usr/local/bin/           && \
   apk del git go make gcc musl-dev linux-headers          && \
   rm -rf /go-ethereum && rm -rf /var/cache/apk/*
 
 EXPOSE 8545
 EXPOSE 30303
 
-ENTRYPOINT ["/geth"]
+ENTRYPOINT ["geth"]