fix: read YAML the way Docker reads it (1.2 booleans)

[lesnik512]

Closes the last known divergence from docker compose. Design: planning/changes/2026-07-14.09-yaml-12-booleans.md.

One root cause, two bugs

PyYAML implements YAML 1.1, where a bare on/off/yes/no is a boolean. Docker parses YAML 1.2, where each is an ordinary string.

Silent corruption, exit 0:

environment:
  SSL: on

podman run ... -e "SSL=true"        # docker: -e SSL=on

The application reads "true" where its author wrote on. Nothing is reported.

A working file rejected:

environment:
  on: 1                             # docker compose renders `on: 1` and runs fine

compose2pod: error: ... key True must be a string

PyYAML resolves the key to the boolean True, and the string-key rule refuses it. Refusing a file Docker runs is the one direction that must never happen for a drop-in replacement.

Neither is repairable downstream: once the loader has turned on into True, the spelling is gone. "on" is not recoverable from True. So it is fixed at the only place it can be — the loader. The CLI now reads YAML with a SafeLoader whose boolean resolver matches only true/false: the YAML 1.2 core schema Docker's parser implements.

I also had a false claim in the architecture doc

It said rejecting a non-string key was "a deliberate divergence from Docker — environment: {3306: db} is valid Compose and Docker accepts it."

Docker refuses it too: non-string key in services.app.environment: 3306. The rule was never a divergence; it matches Docker. That paragraph is corrected here. What Docker never sees is a non-string on: key — because to Docker, on is a string.

Verification

Measured against docker compose config v5.1.2, both axes:

• Values: on→on, off→off, yes→yes, no→no, true→true, false→false. All six match Docker.
• Keys: on/off/yes/no accepted (emit on=1); 3306:/true: refused — by both.

The loader surgery is the risk, so I diffed our loader against plain yaml.safe_load across booleans, ints, floats, nulls, quoted strings, timestamps, octal, anchors and merge keys: the only differences are the intended ones (on/off/yes/no, including where they arrive via an anchor). true/false still resolve as booleans; y/n were already strings in PyYAML and are untouched. The Loader is a genuine SafeLoader subclass.

Scope holds: the core stays stdlib-only (import compose2pod works with PyYAML absent; only cli.py touches yaml), and the JSON path is unchanged — JSON keys are strings by construction.

Three existing tests used on: as their "non-string key" example, which is now a legal string key; they were switched to an int key (3306:), which is non-string under YAML 1.2 and refused by Docker too — so they still test what they claim.

650 tests at 100% coverage; integration green against real podman.

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