Fully automate dev setup with Gitpod (#1868)

* Fully automate dev setup with Gitpod

This commit implements a fully-automated development setup using Gitpod.io, an
online IDE for GitHub and GitLab that enables Dev-Environments-As-Code.
This makes it easy for anyone to get a ready-to-code workspace for any branch,
issue or pull request almost instantly with a single click.

Signed-off-by: justaugustus <foo@agst.us>

.dockerignore

 .github/
 bin/
 tmp/
+.gitpod.yml

.gitpod.yml

+tasks:
+  - init: go get && go build ./... && go test ./... && make
+    command: go run

README.md

@@ -3,6 +3,7 @@
 ![GitHub Workflow Status](https://img.shields.io/github/workflow/status/dexidp/dex/CI?style=flat-square)
 [![Go Report Card](https://goreportcard.com/badge/github.com/dexidp/dex?style=flat-square)](https://goreportcard.com/report/github.com/dexidp/dex)
 [![go.dev reference](https://img.shields.io/badge/go.dev-reference-007d9c?logo=go&logoColor=white&style=flat-square)](https://pkg.go.dev/mod/github.com/dexidp/dex)
+[![Gitpod ready-to-code](https://img.shields.io/badge/Gitpod-ready--to--code-blue?logo=gitpod&style=flat-square)](https://gitpod.io/#https://github.com/dexidp/dex)
 
 ![logo](Documentation/logos/dex-horizontal-color.png)