Add Docker support: multi-stage Dockerfile

Dockerfile

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+# syntax=docker/dockerfile:1
+
+# ---- Build ----
+FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
+WORKDIR /src
+
+COPY NoticeBoard.csproj ./
+RUN dotnet restore NoticeBoard.csproj
+
+COPY . .
+RUN dotnet publish NoticeBoard.csproj -c Release -o /app/publish --no-restore
+
+# ---- Runtime ----
+FROM mcr.microsoft.com/dotnet/aspnet:8.0 AS runtime
+WORKDIR /app
+
+COPY --from=build /app/publish .
+
+# Make sure the folders that need to be writable exist even before a volume is mounted
+RUN mkdir -p /data /app/wwwroot/uploads
+
+ENV ASPNETCORE_HTTP_PORTS=8080 \
+    ASPNETCORE_ENVIRONMENT=Production \
+    DataDirectory=/data
+
+EXPOSE 8080
+VOLUME ["/data", "/app/wwwroot/uploads"]
+
+HEALTHCHECK --interval=30s --timeout=3s --start-period=15s --retries=3 \
+    CMD bash -c 'exec 3<>/dev/tcp/127.0.0.1/8080 && exec 3<&-' || exit 1
+
+ENTRYPOINT ["dotnet", "NoticeBoard.dll"]