docs: add notes regarding remote builds and context

[Tyonar]

Problem to solve

I mistakenly thought I had to copy my Dockerfile and other files relevant to the build to the remote host where I wanted to build the image before the docker_image resource creation.

How to reproduce

$ terraform version
Terraform v1.3.9
on linux_amd64

Summary of Terraform configuration:

terraform {
  required_version = ">= 1.3"
  required_providers {
    docker = {
      source = "kreuzwerker/docker"
      version = "3.0.2"
    }
  }
}

variable "target_host" {
  description = "IP of the VM to target"
  type        = string
  validation {
    condition     = cidrhost("${var.target_host}/32", 0) == var.target_host
    error_message = "target_host must be a valid IP address"
  }
}
provider "docker" {
  host = "ssh://root@${var.target_host}"
}

resource "docker_image" "recursor" {
  name = "recursor"
  build {
    context    = "/opt/recursor"
    tag        = ["recursor:current"]
    dockerfile = "/opt/recursor/Dockerfile"
  }
}

Original error I encountered:

docker_image.recursor: Creating...
docker_image.recursor: Still creating... [10s elapsed]
╷
│ Error: failed to solve with frontend dockerfile.v0: failed to read dockerfile: open /var/lib/docker/tmp/buildkit-mount4152034525/opt/recursor/Dockerfile: no such file or directory
│ 
│ 
│ 
│   with docker_image.recursor,
│   on dns_recursor.tf line 38, in resource "docker_image" "recursor":
│   38: resource "docker_image" "recursor" {
│ 
╵

How to solve

I eventually figured out that the build would work when I set build.context = "." on resource.docker_image.recursor.

I added a few comments in the documentation to clarify that the build context is local even when the provider is configured with a remote host.