Fix docker build

https://github.com/moneroexamples/onion-monero-blockchain-explorer/issues/250

Dockerfile

@@ -43,12 +43,12 @@ ENV BOOST_DEBUG         1
 WORKDIR /root
 
 # Clone and compile monerod with all available threads
-ARG MONERO_BRANCH
 ARG NPROC
 RUN git clone --recursive --branch ${MONERO_BRANCH} https://github.com/monero-project/monero.git \
     && cd monero \
     && test -z "$NPROC" && nproc > /nproc || echo -n "$NPROC" > /nproc && make -j"$(cat /nproc)"
 
+
 # Copy and cmake xmrblocks
 COPY . /root/onion-monero-blockchain-explorer/
 WORKDIR /root/onion-monero-blockchain-explorer/build