aws batch failure: "entrypoint requires the handler name to be the first argument"

I'm getting starting trying to make jobs run via AWS batch. My first submitted job failed with "entrypoint requires the handler name to be the first argument" logged to cloudwatch.

Where should I start in tracing the source of the problem?

Here is my dockerfile:

FROM public.ecr.aws/lambda/python:3.12

# Copy requirements.txt
COPY requirements.txt .

# Install the specified packages
RUN pip install -r requirements.txt

# Copy function code
COPY dojob.py .

# no CMD line --will set the command to be run in the 
# in the job definition via the AWS console.

And here is my job definition:

{
  "jobDefinitionName": "blw-md-batch-job-definition",
  "jobDefinitionArn": "arn:aws:batch:us-east-2:590183906166:job-definition/blw-md-batch-job-definition:1",
  "revision": 1,
  "status": "ACTIVE",
  "type": "container",
  "parameters": {},
  "containerProperties": {
    "image": "590183906166.dkr.ecr.us-east-2.amazonaws.com/blw-md-batch-repo:latest",
    "command": [
      "python3",
      "dojob.py"
    ],
    "jobRoleArn": "arn:aws:iam::590183906166:role/ecsTaskExecutionRole",
    "executionRoleArn": "arn:aws:iam::590183906166:role/ecsTaskExecutionRole",
    "volumes": [],
    "environment": [
      {
        "name": "BLW-MD-JOBID",
        "value": "27e0f6ed-01e4-434d-b438-c9cc59169e3d"
      }
    ],
    "mountPoints": [],
    "ulimits": [],
    "resourceRequirements": [
      {
        "value": "2.0",
        "type": "VCPU"
      },
      {
        "value": "10240",
        "type": "MEMORY"
      }
    ],
    "secrets": [],
    "networkConfiguration": {
      "assignPublicIp": "ENABLED",
      "interfaceConfigurations": []
    },
    "fargatePlatformConfiguration": {
      "platformVersion": "LATEST"
    },
    "runtimePlatform": {
      "operatingSystemFamily": "LINUX",
      "cpuArchitecture": "X86_64"
    }
  },
  "timeout": {
    "attemptDurationSeconds": 1800
  },
  "tags": {
    "blw-make-dataset": ""
  },
  "propagateTags": true,
  "platformCapabilities": [
    "FARGATE"
  ],
  "containerOrchestrationType": "ECS"
}

Thanks!