Revert "[alt-builder] Revert to stable gcc"

This reverts commit f8d537d7

alt-builder/Dockerfile

 FROM amd64/debian:bullseye
 RUN apt-get update && apt-get install -y wget gnupg && \
-    #echo "deb http://apt.llvm.org/unstable/ llvm-toolchain-10 main" >> /etc/apt/sources.list && \
-    #echo "deb-src http://apt.llvm.org/unstable/ llvm-toolchain-10 main" >> /etc/apt/sources.list && \
-    #wget -O llvm.key "https://apt.llvm.org/llvm-snapshot.gpg.key" && apt-key add llvm.key && \
-    #apt-get remove --purge -y wget gnupg && \
-    \
-    apt-get update && \
-    apt-get install -y g++ \
-                       clang++-10 lld-10 \
-                       cmake ninja-build python3 \
-                       jq clang-tidy-10 cppcheck \
-                       libxml2-dev libreadline-dev qtbase5-dev graphviz-dev libjsoncpp-dev libtclap-dev \
-                       libxml2 libreadline8 libqt5widgets5 graphviz libqt5xml5 libjsoncpp1 && \
-    apt-get autoremove -y && \
-    apt-get clean && \
-    rm -rf /var/lib/apt/lists/* && \
-    \
-    for file in /usr/bin/*-10; do ln -sf $file $(echo $file | sed 's/-10$//g'); done
+	#echo "deb http://apt.llvm.org/unstable/ llvm-toolchain-10 main" >> /etc/apt/sources.list && \
+	#echo "deb-src http://apt.llvm.org/unstable/ llvm-toolchain-10 main" >> /etc/apt/sources.list && \
+	#wget -O llvm.key "https://apt.llvm.org/llvm-snapshot.gpg.key" && apt-key add llvm.key && \
+	#apt-get remove --purge -y wget gnupg && \
+	\
+	apt-get update && \
+	apt-get install -y g++ g++-10 clang++-10 lld-10 \
+	                   cmake ninja-build python3 \
+	                   jq clang-tidy-10 cppcheck \
+	                   libxml2-dev libreadline-dev qtbase5-dev graphviz-dev libjsoncpp-dev libtclap-dev \
+	                   libxml2 libreadline8 libqt5widgets5 graphviz libqt5xml5 libjsoncpp1 && \
+	apt-get autoremove -y && \
+	apt-get clean && \
+	rm -rf /var/lib/apt/lists/* && \
+	\
+	for file in /usr/bin/*-10; do ln -sf $file $(echo $file | sed 's/-10$//g'); done