Work-around for Docker bug in BuildKit

[ddsharpe]

Docker has not fixed the bug in the new buildkit for quite some time. And worse, the new install defaults to enabling the new buildkit feature. This Docker bug causes the Docker build step in Image Tool to fail with:
failed to solve with frontend dockerfile.v0: failed to create LLB definition: failed to parse stage name "OS_UPDATE": invalid reference format: repository name must be lowercase

To see the error, the buildkit must be enabled in the Docker preferences/config:
{
"debug": true,
"experimental": false,
"features": {"buildkit": true }
}

[jshum2479]

We probably need to document this change will affect existing user's additional build commands referring to the layer as upper case.

[jshum2479]

Another option is to use these env before image tool to disable buildit. Though I really like the super fast build speed with buildit

export DOCKER_BUILDKIT=0
export COMPOSE_DOCKER_CLI_BUILD=0