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)

tutorcairn/templates/cairn/build/cairn-clickhouse/Dockerfile

@@ -4,4 +4,4 @@ FROM docker.io/clickhouse/clickhouse-server:24.1.8.22
 RUN apt update && apt install -y python3
 COPY ./scripts /scripts
 RUN chmod a+x /scripts/*
-ENV PATH /scripts:${PATH}
+ENV PATH=/scripts:${PATH}