chore(deps): pin docker digests (#602)

Co-authored-by: Renovate Bot <bot@renovateapp.com>

Author: renovate[bot]

Phonebook.Source.PeopleSoft/Dockerfile

@@ -1,11 +1,11 @@
 #See https://aka.ms/containerfastmode to understand how Visual Studio uses this Dockerfile to build your images for faster debugging.
 
-FROM mcr.microsoft.com/dotnet/core/aspnet:3.1-buster-slim AS base
+FROM mcr.microsoft.com/dotnet/core/aspnet:3.1-buster-slim@sha256:a9e160dbf5ed62c358f18af8c4daf0d7c0c30f203c0dd8dff94a86598c80003b AS base
 WORKDIR /app
 EXPOSE 80
 EXPOSE 443
 
-FROM mcr.microsoft.com/dotnet/core/sdk:3.1-buster AS build
+FROM mcr.microsoft.com/dotnet/core/sdk:3.1-buster@sha256:d81d9ab0253c2c578578b2032955984562684c182e3a15dacc3bbdb31260d4e4 AS build
 WORKDIR /src
 COPY ["src/Phonebook.Source.PeopleSoft/Phonebook.Source.PeopleSoft.csproj", "src/Phonebook.Source.PeopleSoft/"]
 COPY ["src/Phonebook.Source.PeopleSoft.Models/Phonebook.Source.PeopleSoft.Models.csproj", "src/Phonebook.Source.PeopleSoft.Models/"]