Correcting docker build to include tag

docker/Dockerfile

@@ -2,6 +2,7 @@ FROM alpine
 LABEL maintainer="Marc Mettke <[email protected]>"
 
 ENV SYSTEM https://github.com/mettke/ssl-cert-authority.git
+ENV TAG v0.3.1
 ADD entrypoint.sh /entrypoint.sh
 ADD healthcheck.sh /healthcheck.sh
 ADD cron /var/spool/cron/crontabs/root
@@ -27,6 +28,7 @@ RUN mkdir -p /var/log/cert/ /run/php/ /sca/ && \
     ln -sf /dev/stderr /var/log/php7/error.log
 RUN apk add git && \
    git clone ${SYSTEM} /sca && \
+   git -C /sca checkout ${TAG} && \
    apk del git && \
    chown -R cert-sync:nogroup /sca/config