Restrict CI on forks, more.

- Don't do container image CI on forks. (We shouldn't be pushing
  container images to ghcr.io from forks!)

- Remove the `if` condition in `deploy_guide.yml`. It has no effct
  because there is an `on: push: branches: - main` condition higher up
  doing the same thing (and in the more typical way).

(These changes were supposed to be in #268, but I failed to commit the
changes before pushing, sigh.)

Author: nnethercote

.github/workflows/container_images.yml

@@ -7,6 +7,8 @@ on:
       - ".github/workflows/container_images.yml"
       - "container/**"
   push:
+    branches:
+      - main
     paths:
       - ".github/workflows/container_images.yml"
       - "container/**"
@@ -162,4 +164,4 @@ jobs:
             $(printf '${{ env.REGISTRY }}/${{ matrix.variance.image }}@sha256:%s ' *)
       - name: Inspect image
         run: |
-          docker buildx imagetools inspect ${{ env.REGISTRY }}/${{ matrix.variance.image }}:${{ steps.meta.outputs.version }}
+          docker buildx imagetools inspect ${{ env.REGISTRY }}/${{ matrix.variance.image }}:${{ steps.meta.outputs.version }}

.github/workflows/deploy_guide.yml

@@ -12,7 +12,6 @@ concurrency:
 jobs:
   deploy:
     # Only run on the main repository, not on forks
-    if: github.repository == 'rust-gpu/Rust-CUDA'
     runs-on: ubuntu-latest
     environment:
       name: github-pages