Add scripts to run shellcheck and shfmt

ci/shellcheck.sh

@@ -0,0 +1,3 @@
+#!/bin/bash
+
+shellcheck "$(find . -type f | grep -v '\.git' | grep '\.sh')"

ci/shfmt.sh

@@ -0,0 +1,3 @@
+#!/bin/bash
+
+docker run --rm -u "$(id -u):$(id -g)" -v "$PWD:/mnt" -w /mnt mvdan/shfmt -l -i 4 .