web: build image in CI, pull it in Terraform instead of building locally

templates/web's docker_image resource used a `build` block, so every
first-use of a new Dockerfile hash triggered a from-scratch build
(including the ~15-20min Rust toolchain compile) right at `terraform
apply` time - i.e. while someone was waiting to create a workspace.

Adds a build-web-image job to coder-templates.yml that builds and pushes
git.octoturge.com/octo-tech/profiles-web:<dockerfile-sha1> to this
instance's container registry, tagged identically to what
docker_image.web now computes and pulls (no build block). provision now
depends on build-web-image so a template never gets pushed pointing at
an image that isn't there yet. Skips the build entirely if that tag's
already in the registry, so an unrelated templates/* change doesn't
pay any cost.

Runs on a new dedicated "docker-build" runner (profiles-web-build),
scoped to just this repo via a repo-level registration token, with
host Docker socket access - deliberately not added to the existing
shared runner-1, which has no such access and stays untouched. Repo is
public, so the pulled image needs no registry auth; the push does, via
a new GITEA_PACKAGE_TOKEN repo secret (write:package scope).

Since CI and this Coder deployment share the same Docker daemon, the
"pull" is normally a same-host cache hit, not a real network pull.

Verified: `terraform validate` passes against the updated
templates/web/main.tf (run directly inside the coder-server container,
which has terraform embedded).

.gitea/workflows/coder-templates.yml

@@ -13,6 +13,19 @@ name: Provision Coder Templates
 #   CODER_URL             e.g. https://code.octoturge.com
 #   CODER_SESSION_TOKEN   a token from `coder tokens create`, ideally under a
 #                          dedicated service account rather than a personal one
+#   GITEA_PACKAGE_TOKEN   a Gitea access token (user Settings > Applications)
+#                          with write:package scope, for pushing templates/web's
+#                          image to this instance's container registry. Only
+#                          the octoturge account's own token is used - login()
+#                          hardcodes that username to match.
+#
+# templates/web builds its Docker image here (build-web-image, on the
+# dedicated "docker-build" runner - see templates/web/main.tf for why: that
+# runner is scoped to this repo only and has host Docker socket access that
+# the shared runner-1 deliberately doesn't). provision then just pulls the
+# tag build-web-image produced, instead of building it itself at
+# `terraform apply` time - keeps the slow Rust toolchain compile off of
+# "someone is waiting to create a workspace".
 
 on:
   push:
@@ -23,7 +36,30 @@ on:
   workflow_dispatch: {}
 
 jobs:
+  build-web-image:
+    runs-on: docker-build
+    container:
+      volumes:
+        - /var/run/docker.sock:/var/run/docker.sock
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Build and push templates/web's image (skips if the tag already exists)
+        run: |
+          set -e
+          TAG="$(sha1sum templates/web/Dockerfile | cut -d' ' -f1)"
+          IMAGE="git.octoturge.com/octo-tech/profiles-web:${TAG}"
+          echo "${{ secrets.GITEA_PACKAGE_TOKEN }}" | docker login git.octoturge.com -u octoturge --password-stdin
+          if docker manifest inspect "$IMAGE" >/dev/null 2>&1; then
+            echo "$IMAGE already in the registry (Dockerfile unchanged), skipping build."
+            exit 0
+          fi
+          docker build -t "$IMAGE" templates/web
+          docker push "$IMAGE"
+
   provision:
+    needs: build-web-image
     runs-on: ubuntu-latest
     env:
       CODER_URL: ${{ secrets.CODER_URL }}