fix: docker build deprecation warnings

With the latest Docker upgrade, we got the following warnings during
build:

	FromAsCasing: 'as' and 'FROM' keywords' casing do not match
	LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format

changelog.d/20240621_170044_regis.md

@@ -0,0 +1 @@
+- [Bugfix] Fix legacy warnings during Docker build. (by @regisb)

tutorxqueue/templates/xqueue/build/xqueue/Dockerfile

@@ -16,7 +16,7 @@ RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
         pkg-config && \
     sed -i '/en_US.UTF-8/s/^# //g' /etc/locale.gen && \
     locale-gen
-ENV LC_ALL en_US.UTF-8
+ENV LC_ALL=en_US.UTF-8
 
 ###### Git-clone xqueue repo ######
 ARG APP_USER_ID=1000
@@ -28,7 +28,7 @@ WORKDIR /openedx/xqueue
 
 ###### Install python venv ######
 RUN python -m venv /openedx/venv
-ENV PATH /openedx/venv/bin:${PATH}
+ENV PATH=/openedx/venv/bin:${PATH}
 # https://pypi.org/project/setuptools/
 # https://pypi.org/project/pip/
 # https://pypi.org/project/wheel/