CI: download shellcheck as static binary (MR 2673)

Adjust to shellcheck removal in Alpine. Currently it's still in edge for
x86_64, but installation fails with:

  ERROR: unable to select packages:
    so:libffi.so.7 (no such package):
      required by: shellcheck-0.7.2-r1[so:libffi.so.7]

Use the official static binary release from shellcheck instead of the
Alpine package. Version 0.7.2 is intentionally used instead of latest
8.0.0, since the latter generates a new warning. Let's unbreak CI first
before adjusting to a newer shellcheck version.

Related: https://lists.alpinelinux.org/~alpine/devel/%3C20211021133615.32f08070%40ncopa-desktop.lan%3E

.ci/shellcheck.sh

@@ -13,6 +13,21 @@ if grep -qr '(CHANGEME!)' *; then
 	exit 1
 fi
 
+# Shellcheck isn't in Alpine anymore, so just download the static binary in CI
+# Related: https://gitlab.alpinelinux.org/alpine/aports/-/issues/10902
+if [ "$DOWNLOAD_SHELLCHECK" = 1 ]; then
+	pkgver="0.7.2"
+	tmpdir="/tmp/shellcheck-$pkgver"
+	url="https://github.com/koalaman/shellcheck/releases/download/v$pkgver/shellcheck-v$pkgver.linux.x86_64.tar.xz"
+	mkdir -p "$tmpdir"
+	if ! [ -e "$tmpdir"/rel.tar.xz ]; then
+		echo "Downloading $url"
+		wget -q -O "$tmpdir"/rel.tar.xz "$url"
+	fi
+	tar -C "$tmpdir" -xf "$tmpdir"/rel.tar.xz
+	export PATH="$tmpdir/shellcheck-v$pkgver/:$PATH"
+fi
+
 # Shell: shellcheck
 sh_files="
 	./main/mdss-fb-init-hack/mdss-fb-init-hack.sh

.gitlab-ci.yml

@@ -56,10 +56,8 @@ shellcheck:
   stage: lint
   <<: *only-default
   image: alpine:edge
-  before_script:
-    - apk -q add shellcheck
   script:
-    - .ci/shellcheck.sh
+    - DOWNLOAD_SHELLCHECK=1 .ci/shellcheck.sh
 
 # aports checks (generic)
 aports-static: