Add build arg for Electrum version

Dockerfile

@@ -1,5 +1,7 @@
 FROM python:3.7-slim-buster
 
+ARG ELECTRUM_VERSION=3.3.8
+
 RUN apt-get update
 
 RUN apt-get install -y git
@@ -7,7 +9,7 @@ RUN git clone git://github.com/spesmilo/electrum.git
 
 WORKDIR electrum
 
-RUN git checkout 3.3.8
+RUN git checkout $ELECTRUM_VERSION
 RUN git submodule update --init
 RUN apt-get install -y libsecp256k1-0
 RUN python3 -m pip install --user . cryptography