fix(docker): Docker python-translation-build

[EmmanuelCbd]

SUMMARY

This PR fix the Docker compiler python translation build.
The build didn't run, this step was just passed, because BUILD_TRANSLATIONS flag was not set into python-translation-compiler layer.

fixes: #32161

TESTING INSTRUCTIONS

Set BUILD_TRANSLATIONS to "true"
run docker build and check into folder superset/translations/xx/LC_MESSAGES
With this fix, .mo files are available.

ADDITIONAL INFORMATION

• Has associated issue: No python translations when running Docker python translation compiler - missing ENV parameter #32161
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[mistercrunch]

There are some intricacies as to how ARG/ENV persist across FROM statements in Dockerfile. I think ENV does persist as long as you build upon the previous base, which is super confusing. We may want to audit the whole file and systematically repeat the ARG/ENV in each layer where they are used... It's not DRY, but may prevent issues/confusion...

[rusackas]

@mistercrunch I almost clicked the merge button here, but held off — do you see that audit as a blocker for this?

[mistercrunch]

Not a blocker, merged!