ci: normalise workflow triggers (#459)

Signed-off-by: izzy <me@insrt.uk>

.github/workflows/book.yml

@@ -1,8 +1,8 @@
 name: Build documentation
+
 on:
   push:
-    branches:
+    branches: [main]
-      - main
 
 jobs:
   deploy:

.github/workflows/docker.yaml

@@ -5,8 +5,6 @@ on:
     tags:
       - "*"
   pull_request:
-    branches:
-      - "main"
     paths:
       - "Dockerfile"
 

.github/workflows/rust.yaml

@@ -2,16 +2,8 @@ name: Rust build, test, and generate specification
 
 on:
   push:
-    paths-ignore:
-      - ".github/**"
-      - "!.github/workflows/rust.yaml"
-      - ".vscode/**"
-      - "doc/**"
-      - ".gitignore"
-      - "LICENSE"
-      - "README"
-  pull_request:
     branches: [main]
+  pull_request:
 
 env:
   CARGO_TERM_COLOR: always