Allow the user to provide an alternative path to devcontainer.json

[szabgab]

Body

What are you proposing?

Currently zed looks for the devcontainer.json file in <ProjectRoot>/.devcontainer/devcontainer.json. It would be nice if the user could provide and alternative path either using a command line option zed --devcontainer ~/my-devcontainer.json or using an environment variable DEVCONTAINER=~/my-devcontainer.json outside of the project folder.

Why does this matter?

I organize online events where we pick some arbitrary open source project and try to contribute to it something relatively simple. For example adding a test, configuring some meta-data, etc. Most of these projects don't have a devcontainer.json and we don't want to add one. We would also prefer not to run the code on the local machine to reduce our risk.

I tried to run zed inside a Docker container, but that does not seem to work #49131. Using devcontainer would be excellent, but it would be much better if we did not have to copy the devcontainer.json file to every project. That would also help to avoid accidentally adding it to git.

See dev-containers.md