Add TODO for netcore version upgrade.

deployment/docker/Dockerfile.arm32v7

@@ -3,6 +3,7 @@ ARG DOTNET_VERSION=3.0
 FROM microsoft/dotnet:${DOTNET_VERSION}-sdk as builder
 WORKDIR /repo
 COPY . .
+#TODO Remove or update the sed line when we update dotnet version.
 RUN export DOTNET_CLI_TELEMETRY_OPTOUT=1 \
  && find . -type f -exec sed -i 's/netcoreapp2.1/netcoreapp3.0/g' {} \; \
  && dotnet clean \

deployment/docker/Dockerfile.arm64v8

@@ -3,6 +3,7 @@ ARG DOTNET_VERSION=3.0
 FROM microsoft/dotnet:${DOTNET_VERSION}-sdk as builder
 WORKDIR /repo
 COPY . .
+#TODO Remove or update the sed line when we update dotnet version.
 RUN export DOTNET_CLI_TELEMETRY_OPTOUT=1 \
  && find . -type f -exec sed -i 's/netcoreapp2.1/netcoreapp3.0/g' {} \; \
  && dotnet clean \