Add corepack command to install yarn

[vilanell000]

Proposed Changes

• When building the development environment using the instructions from the documentation, faild. cannot find the yarn
• modified the Dockerfile to fix

Related Issue(s)

see also https://github.com/wger-project/docker/issues/133#issue-3307275740

Please check that the PR fulfills these requirements

• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Code has been formatted to avoid unnecessary diffs (ruff format && isort .)

Other questions

• Do users need to run some commands in their local instances due to this PR (e.g. database migration, deployment changes)?