Support for env variables ending with _FILE

entrypoint.sh

# Convert all environment variables with names ending in _FILE into the content of
# the file that they point at and use the name without the trailing _FILE.
# This can be used to carry in Docker secrets.
# Source: https://github.com/grafana/grafana-docker/pull/166/files
for VAR_NAME in $(env | grep '^[^=]\+_FILE=.\+' | sed -r "s/([^=]*)_FILE=.*/\1/g"); do
    VAR_NAME_FILE="$VAR_NAME"_FILE
    if [ "${!VAR_NAME}" ]; then
        echo >&2 "ERROR: Both $VAR_NAME and $VAR_NAME_FILE are set (but are exclusive)"
        exit 1
    fi
    echo "Getting secret $VAR_NAME from ${!VAR_NAME_FILE}"
    export "$VAR_NAME"="$(< "${!VAR_NAME_FILE}")"
    unset "$VAR_NAME_FILE"
done