ci: Add ShellCheck workflow

.github/problem-matcher-gcc.json

@@ -0,0 +1,18 @@
+{
+  "problemMatcher": [
+    {
+      "owner": "shellcheck-gcc",
+      "pattern": [
+        {
+          "regexp": "^(.+):(\\d+):(\\d+):\\s(note|warning|error):\\s(.*)\\s\\[(SC\\d+)\\]$",
+          "file": 1,
+          "line": 2,
+          "column": 3,
+          "severity": 4,
+          "message": 5,
+          "code": 6
+        }
+      ]
+    }
+  ]
+}

.github/workflows/shellcheck.yml

@@ -0,0 +1,22 @@
+name: ShellCheck
+
+on:
+  push:
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  shellcheck:
+    name: Lint with shellcheck
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out source code
+        uses: actions/[email protected]
+
+      - name: Add error matcher
+        run: echo "::add-matcher::$(pwd)/.github/problem-matcher-gcc.json"
+
+      - name: Run shellcheck
+        run: shellcheck -o all -s sh -f gcc $(find . -path ./.git -prune -o -type f -executable -print)