Run continuous integration on a custom image

Write a Dockerfile that describes a Debian image with our build
environment set up in it. Write a script that builds and tags the custom
image. Set the continuous integration workflow to run on the custom
image, pulled from Docker's local image store.

Running the examples on the custom image takes the same steps as running
them on a development machine, so we no longer gain anything from
factoring out the `cargo run --example` steps. That means we can get rid
of the `run-examples` action. Since the build environment is now set up
ahead of time, we can get rid of the `setup-rust` action too.

.forgejo/workflows/continuous-integration.yaml

@@ -3,23 +3,46 @@ on:
   push:
     branches: [main]
 jobs:
+  # build the application, reporting success if the build completes
   build:
     runs-on: docker
+    container:
+      image: dyna3:ci
+    defaults:
+      run:
+        working-directory: app-proto
     steps:
       - uses: https://code.forgejo.org/actions/checkout@v4
-      - uses: ./.forgejo/setup-rust
-      - run: rustup target add wasm32-unknown-unknown
-      - run: cargo install trunk
-      - run: cd app-proto; trunk build
+      - run: trunk build
+  
+  # run the automated tests, reporting success if the tests pass
   test:
     runs-on: docker
+    container:
+      image: dyna3:ci
+    defaults:
+      run:
+        working-directory: app-proto
     steps:
       - uses: https://code.forgejo.org/actions/checkout@v4
-      - uses: ./.forgejo/setup-rust
-      - run: cd app-proto; cargo test
+      - run: cargo test
+  
+  # run the Cargo examples, as described here:
+  #
+  #   Karol Kuczmarski. "Add examples to your Rust libraries"
+  #   http://xion.io/post/code/rust-examples.html
+  #
+  # report success if the examples build and run without errors
   run-examples:
     runs-on: docker
+    container:
+      image: dyna3:ci
+    defaults:
+      run:
+        working-directory: app-proto
     steps:
       - uses: https://code.forgejo.org/actions/checkout@v4
-      - uses: ./.forgejo/setup-rust
-      - uses: ./.forgejo/run-examples
+      - run: cargo run --example irisawa-hexlet
+      - run: cargo run --example three-spheres
+      - run: cargo run --example point-on-sphere
+      - run: cargo run --example kaleidocycle