Add Dockerfile and modify gitlab CI to build the docker

Dockerfile

@@ -0,0 +1,21 @@
+FROM mcr.microsoft.com/dotnet/aspnet:9.0-bookworm-slim-amd64 AS base
+USER $APP_UID
+WORKDIR /app
+
+FROM mcr.microsoft.com/dotnet/sdk:9.0-bookworm-slim-amd64 AS build
+ARG BUILD_CONFIGURATION=Release
+WORKDIR /src
+COPY ["mstdnCats.csproj", "./"]
+RUN dotnet restore "mstdnCats.csproj"
+COPY . .
+WORKDIR "/src/"
+RUN dotnet build "mstdnCats.csproj" -c $BUILD_CONFIGURATION -o /app/build
+
+FROM build AS publish
+ARG BUILD_CONFIGURATION=Release
+RUN dotnet publish "mstdnCats.csproj" -c $BUILD_CONFIGURATION -o /app/publish /p:UseAppHost=false
+
+FROM base AS final
+WORKDIR /app
+COPY --from=publish /app/publish .
+ENTRYPOINT ["dotnet", "mstdnCats.dll"]