check for .yalc/ and yalc.lock, and create these if needed

[becky-gilbert]

Fixes #380

The Dockerfiles for API and front-end try to copy .yalc/ and yalc.lock, which are created when experiments are installed. If a site is installed but no experiments have been installed yet, then pushkin prep will produce an error because these files don't exist. This PR fixes the problem by checking for the existence of the directory and file, and creating a placeholder if they do not exist.

Steps to reproduce

• On fix/aws-deployment (the base branch), run yarn workspaces run build to build packages.
• Run pushkin install site (using local pushkin) to install a new site. Do not install any experiments.
• Run pushkin prep. You will see the error:

Dockerfile:4
--------------------
 2 |     
 3 |     COPY ./src /usr/src/app/src
 4 | >>> COPY .yalc /usr/src/app/.yalc/
 5 |     COPY ./package.json ./yarn.lock ./.babelrc ./yalc.lock ./dockerStart.sh /usr/src/app/
 6 |     
--------------------
ERROR: failed to solve: failed to compute cache key: failed to calculate checksum of ref c3479bfb-5945-45c3-92ae-49c72620f001::pn5ialj989lvreg0siwxy5zu8: "/.yalc": not found

• Switch to this branch (fix/docker-copy-error) and repeat the steps above. Running pushkin prep should succeed.
• Run pushkin install experiment to install an experiment, then pushkin prep. The yalc directory and yalc.lock file should not be overwritten with placeholders.

[changeset-bot]

🦋 Changeset detected

Latest commit: 7a704f2

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
pushkin-cli	Patch

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[github-actions]

👋
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[becky-gilbert]

@cherriechang I see that you linked issue #380 to PR #373, but I didn't see the fix there. So this PR will add it to that branch, if you're happy with the solution.

[cherriechang]

It looks good! I checked the behavior of what happens when you run install experiment after the empty .yalc and yalc.lock is created, and they seem to get updated as intended.

[becky-gilbert]

Thanks for the quick review @cherriechang! BTW I plan to add tests for this - just waiting to add them on the refactored version, which will make it much easier.