Fix some more sonar issues.

Run clang-7 build first so that sonar data gets reported earlier, enable nvidia
in travis builds.

Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:latest
+FROM ubuntu:bionic AS base
 RUN apt-get update \
   && DEBIAN_FRONTEND=noninteractive \
   apt-get install -qy --no-install-recommends \
@@ -28,6 +28,8 @@ RUN apt-get update \
       libxnvctrl-dev \
       libiw-dev
 
+FROM base
+
 COPY . /conky
 WORKDIR /conky/build
 ARG X11=yes