Components

A single EC2 instance in the bootstrap VPC, for work that needs a long-lived host.

The instance will be configured with the following:
    a public ip address, so it can reach the internet from the bootstrap subnets
    a security group allowing all outbound traffic and no inbound traffic
    IMDSv2 required
    SSM Session Manager permissions, for shell access without inbound rules
    an encrypted gp3 root volume, deleted with the instance
    a shutdown from within the instance stops it, rather than terminating it

The instance is terminated, and its root volume deleted, when the project is destroyed.
Stopping and starting it outside of a deploy does not change the stack.

instanceName: a name for the instance
    Applied as the instance Name tag.

instanceType: an EC2 instance type, e.g. m7i.4xlarge

machineImage: an SSM parameter name or an AMI id
    An SSM parameter name begins with '/' and is resolved on every deploy, so a
    deploy after the image is updated replaces the instance. An AMI id begins
    with 'ami-' and pins the image.
    Defaults to the current Ubuntu 24.04 LTS amd64 image.

rootDeviceName: the root device name of the machine image
    Defaults to /dev/sda1 (Ubuntu); Amazon Linux uses /dev/xvda.

rootVolumeSize: the root volume size in GiB
    Defaults to 100.

useSpot: true or false
    Run the instance as a persistent Spot Instance, which EC2 stops, rather than
    terminates, when it needs the capacity back. Defaults to false (On-Demand).
    In return for the lower price:
        a start needs Spot capacity for the instance type in the instance's
        availability zone, without it the instance stays stopped
        EC2 may stop a running instance with two minutes' notice, and only EC2
        starts it again, once capacity returns
        changing instanceType replaces the instance
        a deploy that restarts the instance, as a userData change does, fails
        while EC2 holds the instance stopped or has no capacity for it
    A shutdown from within still stops the instance, and its root volume is kept
    while it is stopped. Destroying the project, or replacing the instance, cancels
    its Spot request, so EC2 launches no instance outside the stack. Do not
    terminate the instance by hand: its request launches another, which is
    terminated only when the project is destroyed or the instance next replaced.
    The request is tagged cls:ec2-instance, so it can be found.
    Turning it on or off replaces the instance.

availabilityZone: an availability zone of the bootstrap VPC, e.g. us-east-2b
    Defaults to the zone of the first bootstrap subnet. Spread Spot instances
    across zones so a capacity shortage in one zone does not keep them all
    stopped. Changing it replaces the instance.

userData: [ "command", ... ]
    Shell commands run as root on first boot.

readableParameters: [ "/parameter/name", ... ]
    SSM parameter names the instance may read, including SecureString parameters
    encrypted with the AWS managed key. Keep secrets here, not in userData, which
    is stored in plain text in the stack template.

tags: { key: value, ... }
    Tags to apply to the instance.