Make sure all GCC images export the right gcc/g++ version as CC/CXX

Some of these images have gcc/g++ not aliased to the right
version installed in the images (i.e. the image gcc8 has gcc-8 installed
but gcc is gcc-7, not an alias to gcc-8). Since conan uses CC
environment variable to set the profile settings.compiler, we need
to make sure CC is aliased to the right compiler version

ci/docker/build.sh

@@ -51,14 +51,14 @@ build()
 build lasote/conanclang40    clang40-x86 clang clang++
 build lasote/conanclang50    clang50-x86 clang clang++
 build lasote/conanclang60    clang60-x86 clang clang++
-build lasote/conangcc5-armv7 gcc5-armv7  gcc   g++     CROSS_BUILDING
-build lasote/conangcc5       gcc5-x86    gcc   g++
-build lasote/conangcc6-armv7 gcc6-armv7  gcc   g++     CROSS_BUILDING
-build lasote/conangcc6       gcc6-x86    gcc   g++
-build lasote/conangcc7-armv7 gcc7-armv7  gcc   g++     CROSS_BUILDING
-build lasote/conangcc7       gcc7-x86    gcc   g++
-build lasote/conangcc8-armv7 gcc8-armv7  gcc   g++     CROSS_BUILDING
-build lasote/conangcc8       gcc8-x86    gcc   g++
+build lasote/conangcc5-armv7 gcc5-armv7  gcc-5   g++-5 CROSS_BUILDING
+build lasote/conangcc5       gcc5-x86    gcc-5   g++-5
+build lasote/conangcc6-armv7 gcc6-armv7  gcc-6   g++-6 CROSS_BUILDING
+build lasote/conangcc6       gcc6-x86    gcc-6   g++-6
+build lasote/conangcc7-armv7 gcc7-armv7  gcc-7   g++-7 CROSS_BUILDING
+build lasote/conangcc7       gcc7-x86    gcc-7   g++-7
+build lasote/conangcc8-armv7 gcc8-armv7  gcc-8   g++-8 CROSS_BUILDING
+build lasote/conangcc8       gcc8-x86    gcc-8   g++-8
 
 docker build ${DOCKER_DIR}/clang-format    -t manu343726/tinyrefl:clang-format    && docker push manu343726/tinyrefl:clang-format
 docker build ${DOCKER_DIR}/clang-format-ci -t manu343726/tinyrefl:clang-format-ci && docker push manu343726/tinyrefl:clang-format-ci