Makefile: Add and install default lego.conf

Add a default lego.conf and have it install to /etc.
This file contains good documentation on what each variable configures.

Makefile

@@ -1,4 +1,5 @@
 # Install lego to /usr/bin
 install:
 	install -m 755 lego /usr/bin
+	install -m 644 lego.conf /etc/
 

lego.conf

@@ -0,0 +1,26 @@
+#!/bin/bash
+
+# Lego Configuration File
+
+# Command that should be run inside of containers when using `lego run`
+LEGO_RUN_CMD="/bin/bash"
+
+# List of Docker volume definitions that should be mapped through to all containers started with lego
+# For example, to map /var/lib/foo from host into the container and the bar volume to /var/lib/bar
+# inside a container, it would look like:
+#     LEGO_VOLUMES="/var/lib/foo:/var/lib/foo bar:/var/lib/bar"
+LEGO_VOLUMES=""
+
+# List of Docker containers to ignore when using `lego list`
+# This should be a list of containers that you wouldn't typically want to run a shell inside of,
+# often providing a service to the rest of the sytem.
+# The keyworks in this list are not full container names. All containers containing any of these
+# words will be ignored. For example, `gitlab` in this list will cause the container `gitlab-runner`
+# to be ignored.
+LEGO_CONTAINER_NAME_IGNORE="registry jenkins gitlab"
+
+# List of Docker images to ignore when using `lego list`
+# This should be a list of images that were pulled to provide a service, not a development environment.
+# Again, this list is like `LEGO_CONTAINER_NAME_IGNORE` in that it is not a list of full image names.
+LEGO_IMAGE_NAME_IGNORE="jenkins registry gitlab"
+