add support for opencontainers meta labels

[piksel]

This PR will add some support for the OpenContainers metadata labels, which can provide additional information about updated images in notifications.

Example template usage:

  {{- if .Report -}}
    {{- with .Report -}}
        {{- range .Updated}}
  - {{.Before.Title}} ({{.ImageName}}) was updated from version {{.Before.Version}} to {{.After.Version}} (released {{.After.Created}})
        {{- end -}}
  {{- else -}}
    {{range .Entries -}}{{.Message}}{{"\n"}}{{- end -}}
  {{- end -}}

[codecov]

Codecov Report

Patch coverage: 45.45% and project coverage change: -0.15% ⚠️

Comparison is base (9f60766) 67.60% compared to head (08d2f3e) 67.46%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1729      +/-   ##
==========================================
- Coverage   67.60%   67.46%   -0.15%     
==========================================
  Files          26       26              
  Lines        2380     2388       +8     
==========================================
+ Hits         1609     1611       +2     
- Misses        672      678       +6     
  Partials       99       99              

Files Changed	Coverage Δ
pkg/container/client.go	48.71% <0.00%> (-0.77%)	⬇️
internal/actions/update.go	70.68% <100.00%> (+0.31%)	⬆️

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