Add opts files for gcc and clang

This is easier than setting CMAKE_FLAGS directly in .gitlab-ci.yml and
allows to change options in a central place (or two places).

base-8/Dockerfile

@@ -30,3 +30,4 @@ RUN apt-get update && apt-get dist-upgrade --no-install-recommends --yes \
 RUN adduser --disabled-password --home /duneci duneci
 USER duneci
 WORKDIR /duneci
+COPY opts.clang opts.gcc /duneci

base-8/opts.clang

+CMAKE_FLAGS="-DCMAKE_CXX_COMPILER='/usr/bin/clang++' -DCMAKE_CXX_FLAGS='-O2 -g -Wall'"

base-8/opts.gcc

+CMAKE_FLAGS="-DCMAKE_CXX_FLAGS='-O2 -g -Wall'"

base-9/Dockerfile

@@ -30,3 +30,4 @@ RUN apt-get update && apt-get dist-upgrade --no-install-recommends --yes \
 RUN adduser --disabled-password --home /duneci duneci
 USER duneci
 WORKDIR /duneci
+COPY opts.clang opts.gcc /duneci

base-9/opts.clang

+CMAKE_FLAGS="-DCMAKE_CXX_COMPILER='/usr/bin/clang++' -DCMAKE_CXX_FLAGS='-O2 -g -Wall'"

base-9/opts.gcc

+CMAKE_FLAGS="-DCMAKE_CXX_FLAGS='-O2 -g -Wall'"