Also remove unsupported `LAMBDA_DOCKER_FLAGS` by the Docker SDK client

The AWS CDK requires at least Node v14 aws/aws-cdk@d0a27c1
Trying to bump conservatively to Node v14: https://github.com/localstack/localstack-pro-samples/blob/master/.github/workflows/makefile.yml#L21

Even for v14, maintenance EOL is coming soon: https://github.com/nodejs/Release

Requires more follow up work before migrating to v3:

```
Serverless: Deprecation warnings:

Variables resolver reports following resolution errors:
  - Cannot resolve variable at "resources.Resources.ApiGatewayRestApi.Properties.Name": Value not found at "self" source,
  - Cannot resolve variable at "resources.Outputs.RestApiId.Export.Name": Value not found at "self" source
From a next major this will be communicated with a thrown error.
Set "variablesResolutionMode: 20210326" in your service config, to adapt to new behavior now
More Info: https://www.serverless.com/framework/docs/deprecations/#NEW_VARIABLES_RESOLVER

Resolution of lambda version hashes was improved with better algorithm, which will be used in next major release.
Switch to it now by setting "provider.lambdaHashingVersion" to "20201221".
While it is highly encouraged to upgrade to new algorithm, you can still use the old approach by setting "provider.lambdaHashingVersion" to "20200924".
More Info: https://www.serverless.com/framework/docs/deprecations/#LAMBDA_HASHING_VERSION_V2
```