support/docker/Dockerfile: use correct ENV syntax

buildx correctly complains about our ENV lines:

3 warnings found (use docker --debug to expand):
- LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 21)
- LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 94)
- LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 95)

The ENV syntax is defined as ENV FOO=BAR, not ENV FOO BAR, so adjust the
Dockerfile to match to get rid of this warning.

https://docs.docker.com/reference/dockerfile/#env

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/docker/Dockerfile

@@ -18,7 +18,7 @@ description="Container with everything needed to run Buildroot"
 ARG TAR_VERSION="1.35"
 
 # Setup environment
-ENV DEBIAN_FRONTEND noninteractive
+ENV DEBIAN_FRONTEND=noninteractive
 
 # This repository can be a bit slow at times. Don't panic...
 COPY apt-sources.list /etc/apt/sources.list
@@ -91,5 +91,5 @@ RUN useradd -ms /bin/bash br-user && \
 
 USER br-user
 WORKDIR /home/br-user
-ENV HOME /home/br-user
-ENV LC_ALL en_US.UTF-8
+ENV HOME=/home/br-user
+ENV LC_ALL=en_US.UTF-8