Add squid version as argument

Dockerfile

@@ -5,7 +5,7 @@ ARG SQUID_VERSION
 ENV SQUID_CACHE_DIR=/var/spool/squid \
     SQUID_LOG_DIR=/var/log/squid \
     SQUID_USER=proxy 
-    SQUID_VERSION=${SQUID_VERSION}
+    SQUID_VERSION=$SQUID_VERSION
 
 WORKDIR /tmp
 RUN echo "deb-src http://deb.debian.org/debian buster main contrib" >> /etc/apt/sources.list && apt-get update && apt-get --no-install-recommends -y build-dep squid && apt-get --no-install-recommends -y install wget tar xz-utils libssl-dev libssl1.1 winbind \