tools/docker: enable ccache by default

tools/docker/Dockerfile

@@ -58,6 +58,9 @@ RUN $IDF_PATH/tools/idf_tools.py --non-interactive install required \
   && $IDF_PATH/tools/idf_tools.py --non-interactive install-python-env \
   && rm -rf $IDF_TOOLS_PATH/dist
 
+# Ccache is installed, enable it by default
+ENV IDF_CCACHE_ENABLE=1
+
 COPY entrypoint.sh /opt/esp/entrypoint.sh
 
 ENTRYPOINT [ "/opt/esp/entrypoint.sh" ]