AWS lambda image environment variables not found

Question

I create AWS lambda using container image. However the environment variables defined in YAML is not found.

How I create the container image

1. Use SAM to scaffold, choosing AWS Quick Start Templates > Hello World Example
2. Update template.yaml with environment variables
3. sam build
4. Push to ECR
5. Create lambda function using ECR container image

Here is the YAML I tried

AWSTemplateFormatVersion: "2010-09-09"
Transform: AWS::Serverless-2016-10-31
Description: >
  python3.9

  Sample SAM Template for sam_hello_world

# More info about Globals: https://github.com/awslabs/serverless-application-model/blob/master/docs/globals.rst
Globals:
  Function:
    Timeout: 3
    Environment:
      Variables:
        BUCKET_NAME: "somes3bucketname"

Resources:
  HelloWorldFunction:
    Type: AWS::Serverless::Function # More info about Function Resource: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#awsserverlessfunction
    Properties:
      PackageType: Image
      Architectures:
        - x86_64
    Metadata:
      Dockerfile: Dockerfile
      DockerContext: ./hello_world
      DockerTag: python3.9-v1

AWSTemplateFormatVersion: "2010-09-09"
Transform: AWS::Serverless-2016-10-31
Description: >
  python3.9

  Sample SAM Template for sam_hello_world

# More info about Globals: https://github.com/awslabs/serverless-application-model/blob/master/docs/globals.rst
Globals:
  Function:
    Timeout: 3

Resources:
  HelloWorldFunction:
    Type: AWS::Serverless::Function # More info about Function Resource: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#awsserverlessfunction
    Properties:
      PackageType: Image
      Architectures:
        - x86_64
      Environment:
        Variables:
          BUCKET_NAME: "somes3bucketname"
    Metadata:
      Dockerfile: Dockerfile
      DockerContext: ./hello_world
      DockerTag: python3.9-v1

My app.py

import io
import json
import os

def lambda_handler(event, context):

    print(os.environ)
    print(os.environ["BUCKET_NAME"])
    return {"statusCode": 200}

However, this will throw keyerror because the environment variable is not found by os.environ.

{
  "errorMessage": "'BUCKET_NAME'",
  "errorType": "KeyError",
  "requestId": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
  "stackTrace": [
    "  File \"/var/task/app.py\", line 32, in lambda_handler\n    os.environ[\"BUCKET_NAME\"],\n",
    "  File \"/var/lang/lib/python3.9/os.py\", line 679, in __getitem__\n    raise KeyError(key) from None\n"
  ]
}

Answer 1

I suspect my steps from 3 onwards is wrong.

Basically, when sam build is executed, it builds the container image, but the environment variables are not added into the image itself.

The only time the template.yaml is taken into consideration is when you do sam deploy.