Separate Docker and VM base image versions

[r4victor]

Closes #3962

• Split base_image into docker_base_image and vm_base_image.
• Replaced the old DSTACK_BASE_IMAGE* env vars with two groups:
  • DSTACK_DOCKER_BASE_IMAGE, DSTACK_DOCKER_BASE_IMAGE_VERSION, DSTACK_DOCKER_BASE_IMAGE_UBUNTU_VERSION
  • DSTACK_VM_BASE_IMAGE_VERSION
• VM image name resolution in the AWS, Azure, GCP, and OCI backends now reads settings.DSTACK_VM_BASE_IMAGE_VERSION instead of version.base_image directly.
• Extracted VM images GA workflow from docker.yml into separate vm-images.yml.