Add custom environment docs

docs/custom_tools.md

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+# Custom environments
+
+You may need to run Terraform in a custom environment, if you need additional tools like aws-cli etc.
+These are the options available to you.
+
+## Using the released actions
+
+The published actions run Terraform/OpenTofu in a container using a pre-prepared image.
+The image is based on `debian:bullseye-slim` and is designed to be as small as possible.
+
+It is possible to use the  `TERRAFORM_PRE_RUN` environment variable to customise the environment before running Terraform.
+The command is run using `bash -xeo pipefail`.
+
+For example:
+```yaml
+env:
+  TERRAFORM_PRE_RUN: |
+    # Install latest Azure CLI
+    curl -skL https://aka.ms/InstallAzureCLIDeb | bash
+     
+    # Install postgres client
+    apt-get install -y --no-install-recommends postgresql-client
+```
+
+The downside of this approach is that the `TERRAFORM_PRE_RUN` will be executed by every step, even when the container image is cached.
+
+## Customising the image
+
+Instead of the published, versioned actions you can instead run the actions directly from this repo, or a fork of it.
+
+```yaml
+jobs:
+  plan:
+    runs-on: ubuntu-latest
+    name: Create terraform plan
+    steps:
+      - name: plan
+        uses: dflook/terraform-github-actions/terraform-plan@main
+```
+
+This will build the image from the `image/Dockerfile` in the repo. Any changes to the Dockerfile will be reflected in the runtime environment.
+This is another way to customise the environment. 
+
+The image will be rebuilt for every step, but it does benefit from caching on the actions runner if the image is not changed.
+Typically, it takes 25-30 seconds to build the image on a GitHub hosted runner for the first terraform step in the job.
+Any subsequent terraform steps in the job will use the cached image so will be much faster.
+
+## Using a pre-built custom image
+
+The most efficient way to customise the environment is to build the image once and push it to a registry.
+
+Unfortunately, GitHub actions do not support specifying a custom image for an existing docker based action.
+What you could do instead is fork this repo, and change the `image` in each of the `action.yaml` metadata files to point to your custom image.
+
+This is what the `action.yaml` files would look like when using an image in dockerhub:
+```yaml
+runs:
+  using: docker
+  image: docker://danielflook/terraform-github-actions:v1.37.0
+```
+
+The image must be based on the image built from the dockerfile in this repo. You can use the published images as a starting point to customise, e.g.:
+
+```dockerfile
+FROM danielflook/terraform-github-actions:v1.37.0
+
+RUN curl -skL https://aka.ms/InstallAzureCLIDeb | bash
+RUN apt-get install -y --no-install-recommends postgresql-client
+```
+
+Then you can use the forked actions in your workflow:
+
+```yaml
+jobs:
+  plan:
+    runs-on: ubuntu-latest
+    name: Create terraform plan
+    steps:
+      - name: plan
+        uses: <my_org>/terraform-github-actions/terraform-plan@main
+```