ECR: fix image wrong type for pushed_at

[SoenkeD]

When executing ECR.DescribeImages() and there is an image to describe the Go AWS SDK fails to de-serialize the response with the following message de serialization failed, failed to decode response body, expected PushTimestamp to be a JSON Number, got string instead .
When looking into the code https://github.com/getmoto/moto/blob/master/moto/ecr/models.py line 279 (self.image_pushed_at = str(datetime.now(timezone.utc).isoformat()) ) which obviously is a string. When looking at the AWS API documentation at https://docs.aws.amazon.com/AmazonECR/latest/APIReference/API_DescribeImages.html#API_DescribeImages_ResponseSyntax AWS expects a number (likely a timestamp) so the bug seems to not be in the GO SDK, but in Moto.
Trying the patched version did indeed fix the problem.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e422e95) 95.88% compared to head (80ebe3f) 95.88%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7122   +/-   ##
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  Coverage   95.88%   95.88%           
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  Files         834      834           
  Lines       82202    82202           
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  Hits        78819    78819           
  Misses       3383     3383           

Flag	Coverage Δ
servertests	35.80% <0.00%> (ø)
unittests	95.82% <100.00%> (ø)

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[bblommers]

LGTM - thanks for the fix @SoenkeD!