Use --locked instead of --frozen when running uv sync

[browniebroke]

Description

As per latest recommendations: astral-sh/uv-docker-example#53

Checklist:

• I've made sure that tests are updated accordingly (especially if adding or updating a template option)
• I've updated the documentation or confirm that my change doesn't require any updates

Rationale

The main difference is that --locked assert that the uv.lock will remain unchanged. Production and CI systems should expect that to be the case.