Install tzdata and default TimeZone=UTC in the container

Dockerfile

@@ -14,6 +14,12 @@ RUN dotnet publish NoticeBoard.csproj -c Release -o /app/publish --no-restore
 FROM mcr.microsoft.com/dotnet/aspnet:8.0 AS runtime
 WORKDIR /app
 
+# tzdata is required for TimeZoneInfo.FindSystemTimeZoneById to resolve IANA ids
+# (e.g. "Australia/Melbourne") on Linux — without it, only "UTC" would work.
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends tzdata \
+    && rm -rf /var/lib/apt/lists/*
+
 COPY --from=build /app/publish .
 
 # Make sure the folders that need to be writable exist even before a volume is mounted
@@ -21,7 +27,8 @@ RUN mkdir -p /data /app/wwwroot/uploads
 
 ENV ASPNETCORE_HTTP_PORTS=8080 \
     ASPNETCORE_ENVIRONMENT=Production \
-    DataDirectory=/data
+    DataDirectory=/data \
+    TimeZone=UTC
 
 EXPOSE 8080
 VOLUME ["/data", "/app/wwwroot/uploads"]