Add devcontainer config

(breaking shish/safe down into smaller parts for easier merging)

These configs make it so that one can open the Safe project in a compatible editor (eg vscode) and the IDE will do all the work (eg build, test) in a self-contained container with all the right versions of all the right tools installed. This is particularly useful for Safe, whose build process only runs with specific PHP versions, which may differ from what is installed on the developer's host system.

.devcontainer/Dockerfile

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+FROM debian:stable
+ARG DEBIAN_FRONTEND=noninteractive
+ARG XDEBUG_MODE=coverage
+RUN apt update -y && apt install -y git composer php-cli php-dom php-curl php-xdebug vim
+WORKDIR /app
+CMD cd /app/generator/doc && ./update.sh && cd /app/generator && composer install && php ./safe.php generate && composer cs-fix

.devcontainer/build.sh

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+#!/bin/sh
+cd $(dirname $0)/../
+docker run --rm -v ${PWD}:/app $(docker build -q -f .devcontainer/Dockerfile .)

.devcontainer/devcontainer.json

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+{
+	"name": "Safe PHP",
+	"build": {
+		"context": "..",
+		"dockerfile": "./Dockerfile"
+	},
+
+	"workspaceMount": "source=${localWorkspaceFolder},target=/app,type=bind",
+	"workspaceFolder": "/app"
+}

.devcontainer/run.sh

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+#!/bin/sh
+cd $(dirname $0)/../
+docker run --rm -ti -v ${PWD}:/app $(docker build -q . -f .devcontainer/Dockerfile) /bin/bash