Improved documentation on how to run the Sample

[SamStephens]

When I tried to run the Sample file as per README.md, NO_PREBUILT_LAMBDA=1 npx cdk deploy -a "npx ts-node -P tsconfig.dev.json --prefer-ts-exts test/example.ecr-deployment.ts", I got errors about missing dependencies.

This is because NodeJS, Yarn, and the dependencies Yarn installs, are prerequisites.

$ npm install --global yarn
$ yarn install --check-files --frozen-lockfile

Also I had to run the cdk executable, directly, not via npx:

NO_PREBUILT_LAMBDA=1 cdk deploy -a "npx ts-node -P tsconfig.dev.json --prefer-ts-exts test/example.ecr-deployment.ts"

Lastly, I had to update test/example.ecr-deployment.ts to use the region I've bootstrapped the CDK.

I think the actions here are:

• Documentation on how to install development dependencies. This should probably be its own section, as it doesn't just apply to the Sample file.
• Update the Sample line in README.md to be NO_PREBUILT_LAMBDA=1 cdk deploy -a "npx ts-node -P tsconfig.dev.json --prefer-ts-exts test/example.ecr-deployment.ts" with cdk not called via npx.
• Remove the line in test/example.ecr-deployment.ts that hard codes the region - pick up the region from the users environment instead.

[SamStephens]

Also the sample requires dockerhub credentials as a secret. This isn't documented, and the whole way credential secrets work is confusing.

My take is that we deal with this by commenting out the lines in question, so that by default there's no reliance on secrets and the sample just deploys a locally built image. We can leave the Dockerhub deployment code commented out with a comment explaining it, just like the tarball from s3 code directly below the Dockerhub deployment code.