Fix Docker build (#44)

Dockerfile

@@ -4,13 +4,13 @@ VOLUME /etc/mxisd
 VOLUME /var/mxisd
 EXPOSE 8090
 
+RUN apk update && apk add bash && rm -rf /var/lib/apk/* /var/cache/apk/*
 ADD build/libs/mxisd.jar /mxisd.jar
 ADD src/docker/start.sh /start.sh
-RUN mkdir -p /var/mxisd
 
 ENV JAVA_OPTS=""
 ENV CONF_FILE_PATH="/etc/mxisd/mxisd.yaml"
 ENV SIGN_KEY_PATH="/var/mxisd/sign.key"
 ENV SQLITE_DATABASE_PATH="/var/mxisd/mxisd.db"
 
-CMD [ "/start.sh" ]
+CMD [ "/start.sh" ]