Make script/ entrypoints POSIX sh; add bootstrap and setup

script/docker

@@ -1,9 +1,9 @@
-#!/usr/bin/env bash
+#!/bin/sh
 # script/docker: build the Docker image tagged with the project name.
 # Identical in all repos; the tag comes from script/projectname.
-set -euo pipefail
+set -eu
 
-SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd -P)"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
 ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {