fix(docker): add ecpg to docker image

[villebro]

SUMMARY

When enabling the Postgres validator, the docker-compose returns the following error:

After adding libecpq-dev to the "Final lean image" (which is also installed on the pip install image), the Postgres query validator works correctly:

TESTING INSTRUCTIONS

1. add the following feature flag to docker/pythonpath_dev/superset_config_docker.py:

FEATURE_FLAGS = {
    "SQL_VALIDATORS_BY_ENGINE": {"postgresql": "PostgreSQLValidator"},
}

2. Run docker-compose up
3. Open SQL Lab and write an incorrect query
4. See the validator raise an error correctly

ADDITIONAL INFORMATION

• Has associated issue:
• 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

[kgabryje]

Lgtm