Re-use base image's entrypoint, refactor bootstrap script

[carlzogh]

Description of changes:

1. Refactor net5 and net6 bootstrap scripts to /var/runtime/bootstrap

This is in order to make use of the provided image's bundled /lambda-entrypoint.sh entrypoint. The script also handles deciding whether or not the bootstrap should run through the RIE.

This is what /lambda-entrypoint.sh looks like:

#!/bin/sh
# Copyright 2020 Amazon.com, Inc. or its affiliates. All Rights Reserved.

if [ $# -ne 1 ]; then
  echo "entrypoint requires the handler name to be the first argument" 1>&2
  exit 142
fi
export _HANDLER="$1"

RUNTIME_ENTRYPOINT=/var/runtime/bootstrap
if [ -z "${AWS_LAMBDA_RUNTIME_API}" ]; then
  exec /usr/local/bin/aws-lambda-rie $RUNTIME_ENTRYPOINT
else
  exec $RUNTIME_ENTRYPOINT
fi

2. Allow handler to be specified with _HANDLER

_HANDLER will be used if the first command-line argument is not specified. This is to support running through /lambda-entrypoint.sh which does not pass the handler string in as a command-line argument, but as an environment variable.

3. Add support for exec wrapper scripts

ref. https://docs.aws.amazon.com/lambda/latest/dg/runtimes-modify.html#runtime-wrapper

This is controlled by users through the AWS_LAMBDA_EXEC_WRAPPER environment variable in order to hook into the runtime's start command line arguments.

---

Testing

Built base images locally (tagged as dotnet5 and dotnet6) and used those in Lambda container image functions:

# intermediate build stages omitted
FROM dotnet5

COPY --from=build-image /build/build_artifacts/ /var/task/

CMD ["HelloWorld::HelloWorld.Function::FunctionHandler"]

And invoked successfully both locally and in the cloud.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.