Replace MEILISEARCH_HOST by MEILISEARCH_URL in the docker config

[brunoocasali]

⚠️ This issue is generated, it means the examples and the namings do not necessarily correspond to the language of this repository.
Also, if you are a maintainer, feel free to add any clarification and instruction about this issue.

Sorry if this is already partially/completely implemented, feel free to let me know about the state of this issue in the repo.

Related to meilisearch/integration-guides#214

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Replace the env var naming from the Dockerfile environment:

• MEILISEARCH_HOST to MEILISEARCH_URL

Also, look for occurrences inside of the docker-compose.yml file.

After, run the tests according to the instructions in the CONTRIBUTING.md.

Also, check for occurences in the Github Actions workflows and inside of the source code.