Optimize Docker build files

Dockerfile

@@ -1,9 +1,13 @@
-FROM mcr.microsoft.com/dotnet/core/sdk:2.2-alpine
+FROM mcr.microsoft.com/dotnet/core/sdk:2.2-alpine AS build
 
 WORKDIR /app
-COPY PluralKit.API /app/PluralKit.API
-COPY PluralKit.Bot /app/PluralKit.Bot
-COPY PluralKit.Core /app/PluralKit.Core
-COPY PluralKit.Web /app/PluralKit.Web
-COPY PluralKit.sln /app
-RUN dotnet build
+COPY . /app
+RUN dotnet publish -c Release -o out
+
+FROM mcr.microsoft.com/dotnet/core/runtime:2.2-alpine
+WORKDIR /app
+COPY --from=build /app/PluralKit.*/out ./
+
+ENTRYPOINT ["dotnet"]
+CMD ["PluralKit.Bot.dll"]
+