dockerfile: remove duplicated SSLIB arg statement

SSL library is configured via Dockerfile through an ARG statement.
Previously, it was repeated through each build stage. Remove duplicated
default value by using a global arg and import it without any value set
in the different build stages. This solution has been found on the below
stackoverflow.com issue.

https://stackoverflow.com/questions/53681522/share-variable-in-multi-stage-dockerfile-arg-before-from-not-substituted/53683625

Author: a-denoyelle

Dockerfile

@@ -1,6 +1,8 @@
-FROM ubuntu:20.04 AS builder-ssl
 ARG SSLLIB=QuicTLS-1.1.1
 
+FROM ubuntu:20.04 AS builder-ssl
+ARG SSLLIB
+
 # combined list of dependencies for QuicTLS, AWS-LC
 ENV DEBIAN_FRONTEND noninteractive
 RUN apt-get -y update && apt-get -y install git g++ make gcc wget autoconf libtool cmake curl libasan5
@@ -21,7 +23,7 @@ RUN if [ "$SSLLIB" = "QuicTLS-1.1.1" ]; \
     fi
 
 FROM ubuntu:20.04 AS builder
-ARG SSLLIB=QuicTLS-1.1.1
+ARG SSLLIB
 
 COPY --from=builder-ssl /usr/local/include/openssl/ /usr/local/include/openssl/
 COPY --from=builder-ssl \
@@ -54,7 +56,7 @@ RUN apt-get -y update && apt-get -y install git make gcc liblua5.3-0 liblua5.3-d
   && make install
 
 FROM martenseemann/quic-network-simulator-endpoint:latest
-ARG SSLLIB=QuicTLS-1.1.1
+ARG SSLLIB
 
 # Required for lighttpd
 ENV TZ=Europe/Paris