Add -O2 in armv7 Docker build following bug fix

[Raimo33]

follows #8501

Checklist

Before submitting the PR, ensure the following tasks are completed. If an item is not applicable to your PR, please mark it as checked:

• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.
• Important All PRs must consider how to reverse any persistent changes for tools/lightning-downgrade

[ShahanaFarooqui]

For reference: The issue was fixed with PR #8599.

Ack 4e7183c.