Honor the active docker context when resolving the Docker host (not just DOCKER_HOST) #1069

Proposal

testcontainers should resolve the Docker daemon the way the docker CLI does — by honoring the active docker context — instead of only consulting DOCKER_HOST / ~/.testcontainers.properties and otherwise falling back to docker.from_env()'s default socket.

Why

get_docker_host() (in testcontainers/core/docker_client.py) is:

def get_docker_host() -> Optional[str]:
    return c.tc_properties_get_tc_host() or os.getenv("DOCKER_HOST")

When neither tc.host nor DOCKER_HOST is set, DockerClient.__init__ falls through to docker.from_env(), which connects to the platform-default socket (/var/run/docker.sock). But from_env() — unlike the docker CLI — ignores the active docker context.

On OrbStack, colima, rootless Docker, or a machine where Docker Desktop is installed but stopped, the daemon lives on a non-default socket and /var/run/docker.sock is absent or a dangling symlink. So PostgresContainer(...).start() (really, DockerClient() construction) raises:

docker.errors.DockerException: Error while fetching server API version:
('Connection aborted.', FileNotFoundError(2, 'No such file or directory'))

…even though docker ps works fine, because the CLI follows the active context. This appears to be the root cause behind a recurring class of reports — e.g. #757 (same error, reporter pasted docker context ls) and the Mac networking cases in #159.

To reproduce

# OrbStack (or colima) is the active context; Docker Desktop is installed but
# stopped, so /var/run/docker.sock is a dangling symlink to a non-existent socket.
$ docker context show
orbstack
$ docker ps            # works fine
$ unset DOCKER_HOST
$ python -c "from testcontainers.postgres import PostgresContainer as P; P('postgres:16').start()"
docker.errors.DockerException: Error while fetching server API version:
('Connection aborted.', FileNotFoundError(2, 'No such file or directory'))

Proposed fix

Have get_docker_host() consult the active context as a tier between DOCKER_HOST and the from_env() default, mirroring the CLI's precedence:

1. tc.host (~/.testcontainers.properties)
2. DOCKER_HOST
3. active docker context endpoint ← new
4. from_env() default socket

docker-py already exposes this:

from docker.context import ContextAPI

ctx = ContextAPI.get_current_context()   # reads ~/.docker/config.json -> currentContext
host = ctx.Host if ctx else None         # e.g. unix:///Users/me/.orbstack/run/docker.sock

Resolution failures (older SDK without context support, malformed ~/.docker/config.json, no current context) should be swallowed so behavior falls back to today's from_env().

Bonus: get_docker_socket() (used for Ryuk's socket bind-mount) also routes through from_env(), so honoring the context there too keeps the reaper's socket consistent with the client.

Runtime environment

• testcontainers 4.14.2, docker (docker-py) 7.1.0, Python 3.13
• macOS (Apple Silicon); OrbStack active, Docker Desktop installed-but-stopped

$ docker context ls
NAME            DESCRIPTION                               DOCKER ENDPOINT
default         Current DOCKER_HOST based configuration   unix:///var/run/docker.sock
orbstack *      OrbStack                                  unix:///Users/me/.orbstack/run/docker.sock

Workaround

Exporting DOCKER_HOST from the active context before any testcontainers client is built resolves both the client and the Ryuk socket:

import os
from docker.context import ContextAPI

if not os.getenv("DOCKER_HOST"):
    ctx = ContextAPI.get_current_context()
    if ctx and ctx.Host:
        os.environ["DOCKER_HOST"] = ctx.Host

Happy to open a PR along these lines if the maintainers agree on the approach.