Dockefile/buildspec changes for faster buildtime; Some CF changes

[zabdykadyrova]

This PR contains the following changes:

Docker images (app/Dockerfile, executor/Dockerfile)

• Create the non-root user (app / executor) at the top of the build and run subsequent steps as that user, rather than adding the user and chown -R-ing everything at the end. The new process drastically shortens the backend buildtime.
• Reorder layers for better caching: copy dependency manifests first (package.json/package-lock.json, requirements.txt), install deps, then copy the rest of the source — so dependency layers aren't invalidated on
  every source change.

CodeBuild buildspecs (app/api/buildspec.yml, executor/buildspec.yml)

• Switch from docker build --cache-from (pull :latest) to docker buildx build using a docker-container builder. This portion was duplicated from Rally buildspec configurations.

CloudFormation (cloudformation/templates/1-main.yml, x-parameter-values.md)

• Add a new CreateExecutorS3Bucket parameter (defaults to The S3 Bucket already exists) and wire it into the executor CodePipeline stack's CreateS3Bucket. When testing new deployments of Runway in a different AWS account, the deployment was erroring due to the CreateS3Bucket parameter being used for two different resources.
• Relabel the "ECS Information" parameter group to "Job Executor and ECS Information" and add OAuth2Issuer to the Beanstalk metadata.
• Rename the documented parameter ExternalApiTokenIssuer → OAuth2Issuer in the parameter reference doc.
• Bump template version 2.0.5 → 2.0.6

This was tested in akitson-runway-dev.

[amazon-inspector-ohio]

⏳ I'm reviewing this pull request for security vulnerabilities and code quality issues. I'll provide an update when I'm done

[amazon-inspector-ohio]

✅ I finished the code review, and didn't find any security or code quality issues.

[snyk-io-us]

✅ Snyk checks have passed. No issues have been found so far.

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[ea-mtenhoor]

I think we discussed this in a PR for one of the Shiny apps. Does the app user need to be able to modify these files? If not, we should probably leave root as the owner and just give app read access.

[zabdykadyrova]

This PR was created to change this portion.

[ea-mtenhoor]

I looks like the old adduser command included -H to skip creating a home directory. Why was that removed?