[ecr] DockerImageAsset is not optimal - always pushs existing image to ECR

[zxkane]

DockerImageAsset always tries to push the built image to ECR even if no Dockerfile and image layer changes.

Reproduction Steps

Create a ECR repo via DockerImageAsset

const computeImage = new DockerImageAsset(this, 'ComputeImage', {
            directory: path.join(__dirname, '../assets/compute-task')
        });

Dockerfile

FROM python:3.7-alpine

ADD compute.py /app/compute.py

RUN pip install boto3==1.10.* 

Error Log

console log

Sending build context to Docker daemon 3.584kB
Step 1/3 : FROM python:3.7-alpine
---> 8922d588eec6
Step 2/3 : ADD compute.py /app/compute.py
---> Using cache
---> 605abcbbeadf
Step 3/3 : RUN pip install boto3==1.10.*
---> Using cache
---> c2ab563a903c
Successfully built c2ab563a903c
Successfully tagged 034068960621.dkr.ecr.cn-northwest-1.amazonaws.com.cn/cdk/surfingcomputestatckcomputeimage4b6d3c06:latest
WARNING! Using --password via the CLI is insecure. Use --password-stdin.
Login Succeeded
⌛ Pushing Docker image for cdk.out/asset.91f12123bfdb27cb0ac437167dbea43d2cca31c038944cbd3e557dc2ed52d2e9; this may take a while.
The push refers to repository [034068960621.dkr.ecr.cn-northwest-1.amazonaws.com.cn/cdk/surfingcomputestatckcomputeimage4b6d3c06]
db68905d2f2d: Preparing
4bab69c9387c: Preparing
623223ad637d: Preparing
7b07cfe9774a: Preparing
d376bf36dcd6: Preparing
dcd185b843c6: Preparing
77cae8ab23bf: Preparing
dcd185b843c6: Waiting
77cae8ab23bf: Waiting
d376bf36dcd6: Layer already exists
7b07cfe9774a: Layer already exists
db68905d2f2d: Layer already exists
4bab69c9387c: Layer already exists
623223ad637d: Layer already exists
77cae8ab23bf: Layer already exists
dcd185b843c6: Layer already exists
latest: digest: sha256:5acec7d6726ab11318abed3b9f42357b0f7e97a74490cf91b6e8d59b855c391e size: 1787
034068960621.dkr.ecr.cn-northwest-1.amazonaws.com.cn/cdk/surfingcomputestatckcomputeimage4b6d3c06@sha256:5acec7d6726ab11318abed3b9f42357b0f7e97a74490cf91b6e8d59b855c391e|

Environment

• CLI Version : 1.16.3
• Framework Version:
• OS : Macos X 10.15
• Language : Typescript

Other

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This is 🐛 Bug Report

[eladb]

This has been improved in #5733