Factor out default build agent pools and image names into shared variables

[lbussell]

Related: https://github.com/dotnet/dotnet-docker-internal/issues/4475

With the addition of 1ES pipeline templates, we have a few places where our default pool definitions are overridden or ignored. We should consolidate these into one set of variables for our pools and image names so we don't have any hardcoded values anywhere.

docker-tools/eng/common/templates/stages/dotnet/build-test-publish-repo.yml

Lines 64 to 73 in 5a55648

	# Linux AMD64
	linuxAmd64Pool:
	${{ if ne(parameters.linuxAmd64Pool, '') }}:
	${{ parameters.linuxAmd64Pool }}
	${{ elseif eq(variables['System.TeamProject'], parameters.publicProjectName) }}:
	vmImage: $(defaultLinuxAmd64PoolImage)
	${{ elseif eq(variables['System.TeamProject'], parameters.internalProjectName) }}:
	name: NetCore1ESPool-Internal
	image: 1es-ubuntu-2204
	os: linux

docker-tools/eng/common/templates/variables/common.yml

Lines 46 to 57 in 5a55648

	- name: defaultLinuxAmd64PoolImage
	value: ubuntu-latest
	- name: defaultLinuxArm32PoolImage
	value: null
	- name: defaultLinuxArm64PoolImage
	value: null
	- name: defaultWindows2016PoolImage
	value: vs2017-win2016
	- name: defaultWindows1809PoolImage
	value: windows-2019
	- name: defaultWindows2022PoolImage
	value: windows-2022

docker-tools/eng/pipelines/dotnet-buildtools-image-builder-official.yml

Lines 21 to 27 in 5a55648

	extends:
	template: v1/1ES.Official.PipelineTemplate.yml@1ESPipelineTemplates
	parameters:
	pool:
	name: NetCore1ESPool-Internal
	image: 1es-windows-2022
	os: windows

https://github.com/lbussell/dotnet-docker/blob/9482b39dbd103f56ce42b62cbb14bda00f223c40/eng/pipelines/cg-images.yml#L25-L36

[lbussell]

[Triage] The default*PoolImage variables should stay .NET-independent, and we can continue to use the /dotnet/build-test-publish-repo.yml file to define the 1ES PT compliant images. We should however define variables for the SDL scanning pool so that they can be shared between all of our pipelines:

docker-tools/eng/pipelines/dotnet-buildtools-image-builder-official.yml

Lines 21 to 27 in 5a55648

	extends:
	template: v1/1ES.Official.PipelineTemplate.yml@1ESPipelineTemplates
	parameters:
	pool:
	name: NetCore1ESPool-Internal
	image: 1es-windows-2022
	os: windows

[lbussell]

Marking as wave1 since this is 1ES PT follow-up.