Add docker-compose schema v3.9

[thaJeztah]

This is currently just a copy of the v3.8 schema, in preparation of new features to be added in the new schema.

- Description for the changelog

* Add support for docker-compose schema v3.9

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

@ulyssessouza@silvin-lubecki PTAL

[codecov-io]

Codecov Report

Merging #2073 into master will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## master #2073 +/- ##
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Coverage 56.78% 56.78% =======================================
Files 311 311 Lines 21836 21836 =======================================
Hits 12400 12400 Misses 8520 8520 Partials 916 916

[codecov-io]

Codecov Report

Merging #2073 into master will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## master #2073 +/- ##
=======================================
Coverage 56.78% 56.78% =======================================
Files 311 311 Lines 21836 21836 =======================================
Hits 12400 12400 Misses 8520 8520 Partials 916 916

[thaJeztah]

oh; @silvin-lubecki is on PTO; @vdemeester LGTY? 🤗

[silvin-lubecki]

LGTM

[vdemeester]

LGTM

[ulgens]

This is currently just a copy of the v3.8 schema, in preparation of new features to be added in the new schema.

Why? I know it's been years but it still looks weird to add fake support for a schema version which doesn't exist. Can someone provide a link to docs for 3.9?

[thaJeztah]

Why? I know it's been years but it still looks weird to add fake support for a schema version which doesn't exist.

It's slightly confusing, but this repository was used as a the canonical definition for the 3.x compose schema (which was designed for use with docker stack). The docker compose repository maintained a copy of the definition, and was used to provide documentation for the schemas it supported.

Unfortunately, the compose spec was created using the copy of the schema in the docker/compose repository as starting point (which was slightly behind this repository), which is why some options were not included in the initial implementation of the compose spec (e.g. compose-spec/compose-spec#214)