Use Image Devcontainer Step in GitHub Actions

[metaskills]

This helps ensure that we get/cache the image in a unique job and then have other jobs :needs it.

https://github.com/rails-lambda/lamby_benchmark/commit/c3af7728acad23e9f2ae10558ac781d708201bcc

[jeremiahlukus]

Seems like you already did this if you want to modify let me know