Dockerfile: don't use `root` as default user

Our Dockerfile to build the MeiliSearch's image uses the root user. We are aware this is not a good practice. However, we encountered many issues when trying to change this.

History:

• This PR https://github.com/meilisearch/MeiliSearch/pull/1759 (using the meili user instead of root) was accepted, merged, and released in v0.24.0
• We figured out this led to many issues detailed here: one in the documentation (easily fixed) and one regarding Linux users.
• We tried to fix the Linux issue, without any success
• We reverted the PR and released this in v0.25.0 https://github.com/meilisearch/MeiliSearch/pull/2032

Before implementing this issue, we mean, use a non-root user for our Dockerfile, please ensure you took a look a the whole issue conversation since many things have been tried.