Docker: Use ENV key=value

[Alexis-Loskoutoff]

What does this PR change? What problem does it solve?

Use format "ENV key=value" in dockerfiles instead of legacy "ENV key value" format.
Before these change, an error message was displayed during build

 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 3)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 11)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 12)

Was the change previously discussed in an issue or on the forum?

No

Checklist

• I have added tests for all code changes.
• I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I'm done! This pull request is ready for review.

[MichaelEischer]

Thanks!