Fix docker publish build: copy Workplace.Contracts into image

[JohanO]

Summary

• The publish-docker-image workflow has been failing since commit 455b792, which extracted ProviderCalendarEvent/AgendaDateRange into a new Workplace.Contracts project referenced by Workplace.Web.
• The Dockerfile never copies src/Workplace.Contracts/ into the build context, so dotnet publish fails with CS0246: The type or namespace name 'ProviderCalendarEvent' could not be found.
• Adds the missing COPY steps for Workplace.Contracts (csproj for restore, then full source), mirroring the existing Workplace.ServiceDefaults pattern.

Fixes #19

Test plan

• dotnet publish src/Workplace.Web/Workplace.Web.csproj -c Release succeeds locally with Workplace.Contracts building as a dependency
• CI / publish workflow runs green with the fix

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