This lets you specify docker images when creating the VM. These are a special class of Google instances that are setup for running Docker containers.
gce_vm_container(file = NULL, cloud_init = NULL, shell_script = NULL,
image_family = "cos-stable", image_project = "cos-cloud", ...)file location of a valid cloud-init or shell_script file.
One of file or cloud_init or shell_script must be supplied
contents of a cloud-init file, for example read via readChar(file, nchars = 32768)
contents of a shell_script file, for example read via readChar(file, nchars = 32768)
An image-family. It must come from the image_project family.
An image-project, where the image-family resides.
Other arguments passed to gce_vm_create
A zone operation
file expects a filepath to a https://cloudinit.readthedocs.io/en/latest/topics/format.html configuration file or a valid bash script e.g. has !#/bin/ or #cloud-config at top of file.
image_project will be ignored if set, overriden to cos-cloud.
If you want to set it then use the gce_vm_create function directly that this function wraps with some defaults.
https://cloud.google.com/container-optimized-os/docs/how-to/create-configure-instance - help using cloud-init files