Update base image to gcc:bullseye

Update to GCC 11 on Debian 11 Bullseye as the base image.

Author: kinghuang

Dockerfile

@@ -1,4 +1,4 @@
-FROM gcc:6
+FROM gcc:bullseye as build
 
 WORKDIR /usr/local/src
 COPY . /usr/local/src/hdt-cpp/