docs: add kosa8 to supported Docker environments

[drvt69talati]

Adds a short kosa8 section to "Supported Docker environments", next to Podman.

kosa8 serves a Docker-compatible API on a Unix socket. The section gives the two environment variables needed (DOCKER_HOST and TESTCONTAINERS_RYUK_DISABLED=true) and lists the known limitations.

Tested on macOS (arm64) with testcontainers 4.15.0, docker (Python SDK) 7.2.0, Python 3.12.13 and kosa8 v0.9.10, using only the settings in the new section:

• Passed: RedisContainer, PostgresContainer (SQLAlchemy + psycopg2), KafkaContainer (confluentinc/cp-kafka:7.6.0, produce/consume with kafka-python), and DockerContainer with a log wait and exec
• Fails by design: Ryuk with default settings (kosa8 refuses to share its API socket with a container, hence TESTCONTAINERS_RYUK_DISABLED)

Docs-only change.

🤖 Generated with Claude Code

[drvt69talati]

Updated for kosa8 v0.9.10, retested today on macOS (arm64) with testcontainers 4.15.0: RedisContainer, PostgresContainer and now KafkaContainer pass (kosa8 now supports copying files into containers). Bind mounts work for directories allowed with kosa8 daemon start --allow-mount. Ryuk still has to be disabled (TESTCONTAINERS_RYUK_DISABLED=true), since kosa8 never shares its API socket with a container; the limitations list says so.