Fix unexpected /app shadowing in compose.yaml (#7433)

[ongrid]

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Paths within the container got overwritten by volume mount, which caused inconsistencies with the expected contents of directories populated on buid stage.

For example, CSS and graphics may not load — see issue getredash/redash#7433.

What type of PR is this?

• [ ] Refactor
• [ ] Feature
• [X] Bug Fix
• [ ] New Query Runner (Data Source)
• [ ] New Alert Destination
• [ ] Other

Description

How is this tested?

• [ ] Unit tests (pytest, jest)
• [ ] E2E Tests (Cypress)
• [X] Manually
• [ ] N/A

make compose_build
make create_database
make up

Related Tickets & Documents

#7433

Mobile & Desktop Screenshots/Recordings (if there are UI changes)