Add coder/configure example

Adds a directory configure/ containing example files for publishing
images that make use of Coder's configure feature. If an image contains
a script called "configure" in the directory "/coder", it is
automatically run during environment builds. The main use case is
configurations that depend on the $HOME directory that cannot otherwise
be added to an image's Dockerfile.

configure/Dockerfile.ubuntu

@@ -0,0 +1,5 @@
+FROM ubuntu:latest
+
+RUN apt-get update && apt-get install -y curl
+
+COPY [ "configure", "/coder/configure" ]

configure/configure

@@ -0,0 +1,11 @@
+#!/bin/bash
+
+# Configurations that depend on the $HOME directory
+
+## Install NVM, but only if it's not already installed
+if [ ! -d $HOME/.nvm ]; then
+  echo "Installing NVM"
+  curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.35.3/install.sh | bash
+else
+  echo "NVM is already installed"
+fi