add locale to Dockerfile.base

add ``` ENV LC_ALL=C.UTF-8 ENV LANG=C.UTF-8 ``` because in vapour ``` docker run --rm -v $(pwd)/scripts:/scripts:ro -v $(pwd)/results:/results ghcr.io/mdsumner/gdalcheck/gdalcheck:latest /scripts/check_one.sh vapour /results vapour: WARNING (0 errors, 1 warnings, 0 notes) - that warning is about locale (harmless) "warning_messages": "checking R files for syntax errors ... WARNING\nWarning in Sys.setlocale(\"LC_CTYPE\", \"en_US.UTF-8\") :\n OS reports request to set locale to \"en_US.UTF-8\" cannot be honored", ```