Feature: auto-detect remote DOCKER_HOST from current docker context #1025

Summary

When neither tc.host nor DOCKER_HOST is set, testcontainers-python falls
back to docker-py's default (local unix socket / named pipe). It does not
consult the current docker context.

This is surprising for users who use docker context use <name> to point at a
remote docker daemon (e.g. over SSH or a remote TCP endpoint). The docker
CLI works fine, but testcontainers tries to connect to the local socket and
fails — unless the user manually exports DOCKER_HOST.

For comparison:

• testcontainers-go already supports this — feat: support reading Docker host from the current Docker context testcontainers-go#2810
• testcontainers-java has an open enhancement request — [Enhancement]: Support different docker context testcontainers-java#5837

Reproduction

docker context create remote --docker "host=ssh://user@remote-host"
docker context use remote
unset DOCKER_HOST
make tests   # fails: cannot connect to /var/run/docker.sock

Proposal

Add a fallback in get_docker_host() that reads the current docker context's
host (via docker.context.ContextAPI) when no other source is configured,
while leaving the built-in default context to docker-py's normal handling.

Already working on a PR.