Dockerfile: update to buster

Update the base image to buster-slim and use the default gcc version
(gcc-8) rather than a fixed version.
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>

docker/Dockerfile

-FROM debian:stretch-slim
+FROM debian:buster-slim
 
 ARG DEBIAN_FRONTEND=noninteractive
 
@@ -42,7 +42,7 @@ RUN apt-get update && apt-get install --no-install-recommends -y \
     libudev-dev \
     libwrap0-dev \
     libpci-dev \
-    gcc-6 \
+    gcc \
     git
 
 # Cross-compiler toolchains