remove ENV CC et CXX variables from base docker images

since they prevent to be used as a build arg in final image

.ci/travis/linux/scripts/docker-qgis-build.sh

@@ -29,6 +29,10 @@ pushd build > /dev/null
 
 echo "travis_fold:start:cmake"
 echo "${bold}Running cmake...${endbold}"
+
+export CC=/usr/lib/ccache/clang
+export CXX=/usr/lib/ccache/clang++
+
 cmake \
  -GNinja \
  -DUSE_CCACHE=OFF \

.docker/qgis.dockerfile

@@ -11,7 +11,6 @@ LABEL Description="Docker container with QGIS" Vendor="QGIS.org" Version="1.1"
 
 ARG CC=/usr/lib/ccache/clang
 ARG CXX=/usr/lib/ccache/clang++
-ENV QT_SELECT=5
 ENV LANG=C.UTF-8
 
 COPY . /usr/src/QGIS

.docker/qgis3-build-deps-disco.dockerfile

@@ -127,11 +127,7 @@ RUN echo "nb_NO.UTF-8 UTF-8" > /etc/locale.gen
 RUN echo "en_US.UTF-8 UTF-8" >> /etc/locale.gen
 RUN locale-gen
 
-
 RUN echo "alias python=python3" >> ~/.bash_aliases
 
-ENV CC=/usr/lib/ccache/clang
-ENV CXX=/usr/lib/ccache/clang++
-ENV QT_SELECT=5
 ENV LANG=C.UTF-8
 ENV PATH="/usr/local/bin:${PATH}"

.docker/qgis3-build-deps.dockerfile

@@ -140,16 +140,12 @@ RUN echo "nb_NO.UTF-8 UTF-8" > /etc/locale.gen
 RUN echo "en_US.UTF-8 UTF-8" >> /etc/locale.gen
 RUN locale-gen
 
-
 RUN echo "alias python=python3" >> ~/.bash_aliases
 
 # OTB: download and install otb packages for QGIS tests
 RUN curl -k https://orfeo-toolbox.org/qgis/OTB-Linux64.run -o /tmp/OTB-Linux64.run && sh /tmp/OTB-Linux64.run --target /opt/otb
 ENV OTB_INSTALL_DIR=/opt/otb
 
-ENV CC=/usr/lib/ccache/clang
-ENV CXX=/usr/lib/ccache/clang++
-ENV QT_SELECT=5
 ENV LANG=C.UTF-8
 ENV PATH="/usr/local/bin:${PATH}"