ci: Build container image

This is not pushing it yet to a container registry, but let's make sure
that the image building rule works

Test Plan
=========

CI

.github/workflows/container.yml

@@ -0,0 +1,22 @@
+name: build container image
+on:
+  pull_request:
+  push:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  container:
+    runs-on: ubuntu-22.04
+    permissions:
+      id-token: write
+      contents: read
+
+    steps:
+    - uses: actions/checkout@main
+    - uses: DeterminateSystems/nix-installer-action@main
+    - uses: DeterminateSystems/magic-nix-cache-action@main
+    - name: Build container image
+      run: nix build .#container

.github/workflows/static-build-and-upload.yml

@@ -1,7 +1,7 @@
 name: static build and upload
 on:
-    pull_request:
-    push:
+  pull_request:
+  push:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}

.github/workflows/vmtests.yml

@@ -1,7 +1,7 @@
 name: vmtests
 on:
-    pull_request:
-    push:
+  pull_request:
+  push:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}