Tweak docker base image, approval script, and sudoers file; add Docker section to README

Dockerfile

@@ -1,6 +1,6 @@
 # syntax=docker/dockerfile:1
 
-FROM rust:latest AS base
+FROM rust:1.52.1 AS base
 
 ARG DEBIAN_FRONTEND=noninteractive
 

sample/etc/sudo_pair.prompt.user

@@ -2,6 +2,6 @@ Due to security and compliance requirements, this `sudo` session will require ap
 
 To continue, another human must run:
 
-    docker exec -it %h '%B %u %p'
+    docker exec -it %h %B %u %p
 
 If a suitable human is not available and you have an immediate and urgent need to run this command, you may run the above command to approve your own session. Note that doing so will immediately page an oncall security engineer, so this capability should only be used in the event of an emergency.

sample/etc/sudoers.d/sudo_pair

@@ -1 +1 @@
-nobody ALL = (: games) NOPASSWD: LOG_OUTPUT: ALL
+games ALL = (nobody) NOPASSWD:LOG_OUTPUT:ALL

sudo_pair/README.md

@@ -262,6 +262,56 @@ goal to have a minimal set of dependencies. Currently, those are:
 * [rust-lang-nursery/failure][failure]
 * [dtolnay/thiserror][thiserror]
 
+## Manual Testing in Docker
+
+The behavior of `sudo_pair` can be tested using the included Docker image.
+
+First, build and launch the image. It does not have an entrypoint, so using
+the `-i` flag ensures it stays open, and the `-d` flag detaches from it.
+
+```
+docker build -t sudo-pair . && docker run -id --name sudo-pair sudo-pair
+```
+
+Now, you can attach to the image as the `games` user. In the configuration generated for
+this Docker image, `games` is granted the right to passwordlessly sudo as the `nobody` user;
+in turn, the `nobody` user is a member of the `nogroup` group, which triggers the `sudo_pair`
+enforcement mechanism:
+
+```
+docker exec -u games -it sudo-pair /bin/bash
+```
+
+Now that you're logged in as `games`, you can attempt to sudo as `nobody`, and see the `sudo_pair`
+flow:
+
+```
+sudo -u nobody /bin/bash
+```
+
+You'll be given a prompt like the one below explaining that another user must approve your sudo action:
+
+```
+Due to security and compliance requirements, this `sudo` session will require approval and monitoring.
+
+To continue, another human must run:
+
+    docker exec -it d263b8d24076 /usr/bin/sudo_approve 5 112
+
+If a suitable human is not available and you have an immediate and urgent need to run this command,
+you may run the above command to approve your own session. Note that doing so will immediately page
+an oncall security engineer, so this capability should only be used in the event of an emergency.
+```
+
+In another terminal, you can copy and paste the given command (which will implicitly run as root
+inside the container), and your session as `nobody` will be mirrored to both terminals once approved.
+
+To clean up the existing Docker environment before creating a new build, stop and remove the container:
+
+```
+docker stop sudo-pair && docker rm sudo-pair
+```
+
 ## Contributions
 
 Contributions are welcome! This project should hopefully be small