releng - support arm64 docker images

Describe the bug Docker container fails to run policies on ARM EC2 instance type

To Reproduce

1. Launch an arm instance such as m6g.large

2. Run the following

   docker pull cloudcustodian/c7n:0.9.4.0
   docker run \
     --rm \
     -e AWS_DEFAULT_REGION=us-west-2 \
     -w "/opt/custodian" \
     "cloudcustodian/c7n:0.9.4.0" \
     run \
     --output-dir "output" \
     --region us-west-2 policy_test.yml
   

   Returns

   standard_init_linux.go:211: exec user process caused "exec format error"
   

Expected behavior A clear and concise description of what you expected to happen.

Background (please complete the following information):

• OS: arm architecture on m6g.large
• Python Version: container python
• Custodian Version: 0.9.4
• Cloud Provider: aws
• Policy: N/A
• Traceback: See above
• custodian version --debug output: see container

Additional context Works if I switch back to x64 architecture using m5.large

[kapilt]

grpc now supports arm.

[kapilt]

i thought this issue was handy re producing multi-arch builds within github actions https://github.com/actions/cache/issues/260

looking through tools/dev/dockerpkg.py it seems like we should be able to just replace the guts of build_image and push_image with subprocess calls to buildx