update dockerfile image

src/Services/Identity/Identity.API/Dockerfile

@@ -1,8 +1,8 @@
-FROM mcr.microsoft.com/dotnet/core/aspnet:2.2 AS base
+FROM mcr.microsoft.com/dotnet/core/aspnet:3.0-buster-slim AS base
 WORKDIR /app
 EXPOSE 80
 
-FROM mcr.microsoft.com/dotnet/core/sdk:2.2 AS build
+FROM mcr.microsoft.com/dotnet/core/sdk:3.0-buster AS build
 WORKDIR /src
 
 COPY scripts scripts/