Add missing gdiplus libraries to aspnetcore docker image

Dockerfile

FROM mcr.microsoft.com/dotnet/core/aspnet:3.1-buster-slim AS base

RUN apt-get update \
    && apt-get install -y --allow-unauthenticated \
        libc6-dev \
        libgdiplus \
        libx11-dev \
     && rm -rf /var/lib/apt/lists/*
     
# your other stuff below