Dependabot and precommit-bot do not get along

[scottyhq]

It's a minor issue, but I just noticed that:

1. dependabot updates the docker image for codespaces
2. precommit automatically adds a commit to remove end-of-line space
3. dependabot adds a note: "A newer version of pangeo/base-image exists, but since this PR has been edited by someone other than Dependabot I haven't updated it"

#261

I don't think we need to update this image all that often, maybe once every several months, just flagging it.