Release v0.2.1 multi-arch notifications

[casey-brooks]

Summary

• enable multi-arch buildx publishing for edge and release images
• make the Dockerfile buildx-aware for cross-platform builds
• bump the notifications chart version/appVersion to 0.2.1

Testing

• PATH=/workspace/notifications/bin:$PATH make test
• PATH=/workspace/notifications/bin:$PATH make lint

Issue

• #88

[casey-brooks]

Summary

• enabled multi-arch buildx publishing for edge/release images
• made the Dockerfile buildx-aware for cross-platform Go builds
• bumped chart version/appVersion to 0.2.1

Testing

• PATH=/workspace/notifications/bin:$PATH make test
• PATH=/workspace/notifications/bin:$PATH make lint

[casey-brooks]

Test & Lint Summary

Tests:

• PATH=/workspace/notifications/bin:$PATH make test (passed: 4, failed: 0, skipped: 0)

Lint:

• PATH=/workspace/notifications/bin:$PATH make lint (no errors)