Give Terraform CI tailnet access for the Proxmox provider

The bpg/proxmox provider has to reach london-a's API and node, which only
live on the tailnet, during plan and apply. Add a setup-tailnet composite
action (Tailscale via the CI OAuth client + the deploy SSH key in an agent)
and use it in the terraform plan/apply and validate workflows. Pin the
provider's node SSH address to london-a's Tailscale IP so it isn't reached
via the API-reported LAN address.

.github/actions/setup-tailnet/action.yml

@@ -0,0 +1,35 @@
+name: Set up tailnet + SSH
+description: >-
+  Join Tailscale and load the deploy SSH key into an agent, so tofu's Proxmox
+  (bpg) provider can reach london-a's API and node over the tailnet.
+
+inputs:
+  tailscale-client-id:
+    required: true
+  tailscale-audience:
+    required: true
+  ssh-private-key:
+    required: true
+
+runs:
+  using: composite
+  steps:
+    - name: Set up Tailscale
+      uses: tailscale/github-action@v4
+      with:
+        oauth-client-id: ${{ inputs.tailscale-client-id }}
+        audience: ${{ inputs.tailscale-audience }}
+        tags: tag:ci
+
+    - name: Load SSH key into agent
+      shell: bash
+      run: |
+        mkdir -p ~/.ssh
+        echo "${{ inputs.ssh-private-key }}" > ~/.ssh/id_ed25519
+        chmod 600 ~/.ssh/id_ed25519
+        eval "$(ssh-agent -s)"
+        ssh-add ~/.ssh/id_ed25519
+        # Persist the agent for later steps (bpg uses SSH_AUTH_SOCK).
+        echo "SSH_AUTH_SOCK=$SSH_AUTH_SOCK" >> "$GITHUB_ENV"
+        echo "SSH_AGENT_PID=$SSH_AGENT_PID" >> "$GITHUB_ENV"
+        ssh-keyscan -H 100.122.180.98 >> ~/.ssh/known_hosts 2>/dev/null || true

.github/workflows/terraform.yml

@@ -34,6 +34,14 @@ jobs:
       - name: Set backend credentials
         uses: ./.github/actions/tofu-backend-creds
 
+      # Proxmox (bpg) provider reaches london-a over the tailnet.
+      - name: Set up tailnet + SSH
+        uses: ./.github/actions/setup-tailnet
+        with:
+          tailscale-client-id: ${{ secrets.TAILSCALE_CLIENT_ID }}
+          tailscale-audience: ${{ secrets.TAILSCALE_AUDIENCE }}
+          ssh-private-key: ${{ secrets.SSH_PRIVATE_KEY }}
+
       - name: tofu init
         working-directory: terraform/
         run: tofu init
@@ -69,6 +77,14 @@ jobs:
       - name: Set backend credentials
         uses: ./.github/actions/tofu-backend-creds
 
+      # Proxmox (bpg) provider reaches london-a over the tailnet.
+      - name: Set up tailnet + SSH
+        uses: ./.github/actions/setup-tailnet
+        with:
+          tailscale-client-id: ${{ secrets.TAILSCALE_CLIENT_ID }}
+          tailscale-audience: ${{ secrets.TAILSCALE_AUDIENCE }}
+          ssh-private-key: ${{ secrets.SSH_PRIVATE_KEY }}
+
       - name: tofu init
         working-directory: terraform/
         run: tofu init

.github/workflows/validate-terraform.yml

@@ -61,6 +61,15 @@ jobs:
         if: github.actor != 'dependabot[bot]'
         uses: ./.github/actions/tofu-backend-creds
 
+      # Proxmox (bpg) provider reaches london-a over the tailnet during plan.
+      - name: Set up tailnet + SSH
+        if: github.actor != 'dependabot[bot]'
+        uses: ./.github/actions/setup-tailnet
+        with:
+          tailscale-client-id: ${{ secrets.TAILSCALE_CLIENT_ID }}
+          tailscale-audience: ${{ secrets.TAILSCALE_AUDIENCE }}
+          ssh-private-key: ${{ secrets.SSH_PRIVATE_KEY }}
+
       - name: tofu init
         if: github.actor != 'dependabot[bot]'
         working-directory: terraform/

terraform/providers.tf

@@ -60,8 +60,14 @@ provider "proxmox" {
   insecure  = true # self-signed PVE cert
 
   # Uploading the cloud-init snippet needs node-level access; SSH to root@london-a.
+  # Pin the node's SSH address to its Tailscale IP (the API-reported LAN IP isn't
+  # reachable from CI runners on the tailnet).
   ssh {
     agent    = true
     username = "root"
+    node {
+      name    = "london-a"
+      address = "100.122.180.98"
+    }
   }
 }