Sample of buildspec.yml for AWS CodeBuild that builds a Docker image from code and push it to ECR to be deployed via CodePipeline

buildspec.yml

version: 0.2
env:
  variables:
    #
    # NOTE: You need to replace these values!
    #
    ACCOUNT_NUMBER: "REPLACE_ME"
    ECR_REGION: "REPLACE_ME"
    ECR_REPOSITORY_NAME: "REPLACE_ME"
    ECS_CONTAINER_NAME: "REPLACE_ME"
phases:
  install:
    runtime-versions:
      docker: 18
  pre_build:
    commands:
      - echo Logging in to Amazon ECR...
      - aws --version
      - $(aws ecr get-login --region $ECR_REGION --no-include-email)
      - REPOSITORY_URI=$ACCOUNT_NUMBER.dkr.ecr.$ECR_REGION.amazonaws.com/$ECR_REPOSITORY_NAME
      - COMMIT_HASH=$(echo $CODEBUILD_RESOLVED_SOURCE_VERSION | cut -c 1-7)
      - IMAGE_TAG=${COMMIT_HASH:=latest}
  build:
    commands:
      - echo Build started on `date`
      - echo Building the Docker image...          
      - docker build -t $REPOSITORY_URI:latest .
      - docker tag $REPOSITORY_URI:latest $REPOSITORY_URI:$IMAGE_TAG
  post_build:
    commands:
      - echo Build completed on `date`
      - echo Pushing the Docker images...
      - docker push $REPOSITORY_URI:latest
      - docker push $REPOSITORY_URI:$IMAGE_TAG
      - echo Writing image definitions file...
      - printf '[{"name”:"$ECS_CONTAINER_NAME","imageUri":"%s"}]' $REPOSITORY_URI:$IMAGE_TAG > imagedefinitions.json
artifacts:
  files: imagedefinitions.json

Did a few changes in the gist. Hopefully the updated version makes it easier to understand and use.