feat: support build.args list form and add build --build-arg flag

[OrtegaMatias]

Summary

Completes build.args support end-to-end:

• Fixes DecodingError.typeMismatch in build args #136 — build.args in list form (- KEY=VALUE) threw DecodingError.typeMismatch: expected Mapping ... found Node and crashed the entire up. Build now decodes both the map form and the list form.
• Closes build command: add support for --build-arg flag #69 — adds a repeatable --build-arg KEY=VALUE flag to the build subcommand, matching docker compose build.

Details

#136 — list-form build.args

Build.args only decoded [String: String] (map form), so this valid Compose file crashed at decode time:

nginx:
  build:
    context: nginx
    args:
      - DIR=development        # ← list form

Build now accepts map or list, reusing the exact same list-parsing semantics environment: already uses. To avoid duplicating that logic, the body of Service.parseEnvironmentList is lifted into a shared parseComposeKeyValueList(_:) helper that both environment: and build.args: call — split on the first = (so values keep internal =), and a bare KEY inherits from the environment.

#69 — build --build-arg

container-compose build --build-arg TOKEN=$TOKEN --build-arg ENV=prod

Previously failed with Unknown option '--build-arg'. Now, per Docker parity:

• compose-file args: values are interpolated against the environment (unchanged),
• CLI --build-arg values are passed through as-is and win on key conflicts,
• a bare --build-arg KEY inherits its value from the environment.

The merge is a small pure function mergedBuildArgs(fileArgs:cliArgs:), unit-tested independently of the daemon.

Testing (TDD)

Written test-first. Full static suite is green locally:

swift test --filter Container_Compose_StaticTests
✔ Test run with 243 tests in 21 suites passed

New static tests:

• list-form build.args decode + the exact DecodingError.typeMismatch in build args #136 repro at the DockerCompose level
• list-form value with an internal = (DSN) round-trips
• --build-arg flag parsing (repeated + default-empty)
• CLI-overrides-file precedence, and non-colliding CLI+file args both kept

No changes to the dynamic (daemon) tests.

Scope

--build-arg is added only to the build subcommand (parity with Docker). environment: behavior is unchanged (its existing list-form tests still pass after the shared-helper extraction).