adds a dockerfile and a build action

.github/workflows/docker-build-umbraco-image.yml

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+name: Docker Build Umbraco Image
+
+on:
+  push:
+    branches: [ "main", "stage", "develop" ]
+  pull_request:
+    branches: [ "main", "develop" ]
+
+jobs:  
+  build-umbraco-container:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v3
+    - name: Build Umbraco Docker image
+      run: docker build . --file ./Dockerfile --tag my-image-name:$(date +%s)

Dockerfile

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+# syntax=docker/dockerfile:1
+
+FROM mcr.microsoft.com/dotnet/sdk:8.0 as build-env
+WORKDIR /src
+COPY ["ContainerRegistryTest/ContainerRegistryTest.csproj", "."]
+
+RUN dotnet restore
+COPY . .
+RUN dotnet publish ContainerRegistryTest/ContainerRegistryTest.csproj --configuration Release --output /publish
+
+FROM mcr.microsoft.com/dotnet/sdk:8.0 as runtime-env
+WORKDIR /publish
+COPY --from=build-env /publish .
+ENV ASPNETCORE_URLS "http://+:80"
+EXPOSE 80
+ENTRYPOINT [ "dotnet", "ContainerRegistryTest.dll"]