Update docker-compose to support Docker v1.6+

[Lycol50]

Without this, docker-compose will output:
ERROR: The Compose file './docker-compose.yml' is invalid because:
Unsupported config option for services: 'mailserver'

Learn More: https://stackoverflow.com/questions/36724948/docker-compose-unsupported-config-option-for-services-service-web

Description

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change that does improve existing functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[casperklein]

The version field is not required and only informational. See my comment here.

If you get an error, you're probably using a very old docker-compose version. The current version is 2.9.0.

[polarathene]

Closing as omitting version is considered best practice.

The failure you encountered tells us that your docker-compose is over 2 years old, please update it instead of trying to workaround the problem.

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More Info

This is a recurring issue users run into and attempt to "fix": #2643 (comment)

Since the docker-compose 1.27 release (Aug 2020), the version option was made optional. If you are using Ubuntu 20.04 LTS, you may have an outdated docker-compose package (1.25); if so upgrade to Ubuntu 22.04 LTS or follow Dockers official docs to install newer version.

Note that in PR 2643, the user reported using Ubuntu 22.04, but their docker-compose was 1.24 (2019 release), presumably they upgraded Ubuntu, but their package was not updated (may have been installed manually at the time?). If you do have trouble updating, another user shared their troubleshooting experience which may help you.

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For reference:

• June 2020 - Compose spec was updated to make version optional. See the relevant spec section Version top-level element (latest) for more details. An additional reference comment by the author of the change notes version was not providing any value. You can also see the referenced issue of the PR that made it optional for in-depth discussion that came to that decision.
• July 2020 - docker-compose was updated with optional version support, part of the 1.27 (Aug 2020) release, shipped with Ubuntu 21.10 release (the 20.04 LTS release lacks it AFAIK, but should no longer be an issue with 22.04 LTS availability).
• April 2022 - Docker blog announces deprecation of docker-compose 1.x series (last release 1.29.2 was May 2021), EOL in 2023. docker-compose 2.x series is run with docker compose (no dash), unless the setup has aliased docker-compose.

There may also be some issues for swarm users, but we do not support that type of deployment.