fix(context): use default config dir when config.json is missing

[immanuwell]

Fixes #2622.

This fixes context lookup when Docker config exists only as the default dir, with no config.json yet.
Right now docker-py falls back to ./contexts, which is kinda wrong, and set_current_context() blows up instead of creating config.json.

Repro:

1. Make sure ~/.docker/config.json does not exist.
2. Create a context with Docker CLI, so ~/.docker/contexts/... exists.
3. Run:

from docker.context import ContextAPI
print(ContextAPI.contexts())
ContextAPI.set_current_context("my-context")

Before:

• ContextAPI.contexts() only finds default
• ContextAPI.set_current_context() fails
• creating a context can write ./contexts in the cwd, not great

After:

• context paths resolve under the default Docker config dir
• set_current_context() creates config.json if needed

Checks:

• uv run --with pytest pytest tests/unit/context_test.py tests/unit/utils_config_test.py -q
• broader unit checks around this area pass too, the only extra failures I hit on current main were pre-existing timezone-sensitive tests