Fix deprecated Dockerfile ENV format (#31450) (#31452)

Fix deprecated Dockerfile ENV format (#31450) (#31452)

Backport #31450 by @silverwind

See
https://docs.docker.com/reference/build-checks/legacy-key-value-format/.
Fixes these warnings seen during the docker build:

```
 4 warnings found (use --debug to expand):
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 5)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 9)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 75)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 76)
 ```

Introduced in: https://github.com/moby/buildkit/pull/4923

Co-authored-by: silverwind <me@silverwind.io>