[docker] Fix docker-init missing FLASK_APP

[dpgaspar]

CATEGORY

Choose one

• Bug Fix

SUMMARY

New flask fab ... cli needs FLASK_APP env var to be set

TEST PLAN

Run:
docker-compose build
docker-compose run --rm superset ./docker-init.sh

ADDITIONAL INFORMATION

• Has associated issue: Could not locate a Flask application #7707

[codecov-io]

Codecov Report

Merging #7709 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7709   +/-   ##
=======================================
  Coverage   65.76%   65.76%           
=======================================
  Files         459      459           
  Lines       21909    21909           
  Branches     2408     2408           
=======================================
  Hits        14409    14409           
  Misses       7379     7379           
  Partials      121      121

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