ci: add shellcheck to github actions

Note that

1. No golang caching is required, since we do not build any go code,
   and it does not look like it make sense to cache a single binary
   we donwload from github.

2. Use a problem matcher, so the PRs will be annotated with
   shellcheck warnings.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.github/workflows/validate.yml

@@ -45,6 +45,30 @@ jobs:
     - name: shfmt
       run: make shfmt
 
+  shellcheck:
+    name: shellcheck
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: vars
+        run: |
+          echo 'VERSION=v0.7.1' >> $GITHUB_ENV
+          echo 'BASEURL=https://github.com/koalaman/shellcheck/releases/download' >> $GITHUB_ENV
+          echo 'SHA256SUM=1e8499d6f90481bfa2af7a4dce86dd9bcba5c0fbd88bf4793b34ec679c8bb98a' >> $GITHUB_ENV
+          echo ~/bin >> $GITHUB_PATH
+      - name: install shellcheck
+        run: |
+          mkdir ~/bin
+          curl -sSfL $BASEURL/$VERSION/shellcheck-$VERSION.linux.x86_64.tar.xz |
+            tar xfJ - -C ~/bin --strip 1 shellcheck-$VERSION/shellcheck
+          sha256sum ~/bin/shellcheck | grep -q $SHA256SUM
+          # make sure to remove the old version
+          sudo rm -f /usr/bin/shellcheck
+      - uses: lumaxis/shellcheck-problem-matchers@v1
+      - name: shellcheck
+        run: |
+          make shellcheck
+
   deps:
     name: deps
     runs-on: ubuntu-latest