chore: Update Arm64 base image to use a pinned image. (#1724)

Author: nrcventura

src/Agent/NewRelic/Profiler/linux/Arm64Dockerfile

@@ -1,7 +1,8 @@
 # This builds an Ubuntu image, clones the coreclr github repo and builds it.
 # It then sets up the environment for compiling the New Relic .NET profiler.
 
-FROM ubuntu:18.04
+# ubuntu:18.04 - multi-platform image
+FROM ubuntu@sha256:152dc042452c496007f07ca9127571cb9c29697f42acbfad72324b2bb2e43c98 
 
 RUN apt-get update -q -y
 RUN apt-get install -q -y \