Why is the ecr image for lambda built with codebuild not working properly?

Question

I use Codebuild to deploy ecr image for lambda.

CodeCommit Repo:

CodeBuild's buildspec.yml:

version: 0.2

env:
  variables:
    repo_uri: "123456789100.dkr.ecr.ap-northeast-2.amazonaws.com/my-ecr-repo:latest"
    ecr_repo: "my-ecr-repo"
    region: "ap-northeast-2"
phases:
  build:
    commands:
      - aws ecr get-login-password --region ap-northeast-2 | docker login --username AWS --password-stdin 12345678900.dkr.ecr.ap-northeast-2.amazonaws.com
      - docker build -t ${ecr_repo} . --build-arg REGION=${region}
      - docker tag ${ecr_repo} ${repo_uri}
      - docker push ${repo_uri}

Dockerfile:

FROM public.ecr.aws/lambda/python:3.9
    
COPY main/app.py ${LAMBDA_TASK_ROOT}

RUN pip3 install pandas
RUN pip3 install pymysql

ENV PYTHONUNBUFFERED=TRUE

CMD [ "app.lambda_handler" ]

When i start the build, it takes only 3 minutes, which is about 8 times less than if i uploaded an ecr image without using CodeBuild.

Also, When I use this ecr image for lambda, below error is shown:

RequestId: 614fa6a4-2dad-4c5a-8527-8d060b80f86d Error: fork/exec /lambda-entrypoint.sh: exec format error
Runtime.InvalidEntrypoint

NOTE. Already seen that there is a function called lambda_handler inside the app.py file.

NOTE. If there's anything else that I can do to fix this, please comment.