dockerfile: use debian:buster and upgrade to v1.4.0
-FROM debian:stretch as builder
+FROM debian:buster as builder
# Fluent Bit version
ENV FLB_MAJOR 1
-ENV FLB_MINOR 3
+ENV FLB_MINOR 4
ENV FLB_PATCH 0
-ENV FLB_VERSION 1.3.0
+ENV FLB_VERSION 1.4.0
ENV DEBIAN_FRONTEND noninteractive
