build: env variables improvements

[sebastianmusial]

PR Type

What kind of change does this PR introduce?

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [x] Build related changes
• [ ] CI related changes
• [ ] Documentation content changes
• [ ] Other... Please describe:

What is the current behavior?

• We had to set up the app URL in multiple places
• hard configuration for multiple instances

What is the new behavior?

• added new env variable (.env) - APP_URL which represents the main app URL for the iframe

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No