dockcross: make the default image specific to each compiler

linux-armv6/Dockerfile

@@ -39,5 +39,7 @@ RUN mkdir rpi_tools && cd rpi_tools && git init && git remote add -f origin http
 ENV QEMU_LD_PREFIX ${CROSS_ROOT}/libc
 ENV QEMU_SET_ENV "LD_LIBRARY_PATH=${CROSS_ROOT}/lib:${CROSS_ROOT}/libc/lib/${CROSS_TRIPLE}/"
 
+ENV DEFAULT_DOCKCROSS_IMAGE thewtex/cross-compiler-linux-armv6
+
 COPY Toolchain.cmake ${CROSS_ROOT}/
 ENV CMAKE_TOOLCHAIN_FILE ${CROSS_ROOT}/Toolchain.cmake