Dockerfile: use gcc-7 which has retpoline support

The default gcc v8 in Debian Buster causes this kernel build error:

  You are building kernel with non-retpoline compiler.

So we need to use gcc-7 instead which has retpoline support.
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>

docker/Dockerfile

+# no comment again
+
 FROM debian:buster-slim
 
 ARG DEBIAN_FRONTEND=noninteractive
@@ -21,7 +23,7 @@ RUN apt-get update && apt-get install --no-install-recommends -y \
     debhelper \
     dh-exec \
     flex \
-    gcc \
+    gcc-7 \
     git \
     gcc-multilib \
     kernel-wedge \