Multiple drush versions

Fixes #2 by adding a Dockerfile for each stable version of drush available to install with composer (i.e.: greater than 6.0.0).

@RobLoach, once you merge this, you'll have to fix some stuff at https://hub.docker.com/r/drush/drush/~/settings/automated-builds/ ...

In the unlabelled table with columns "Type", "Name", "Dockerfile Location", and "Docker Tag Name":

Fix the row whose "Docker Tag Name" = latest by setting "Dockerfile Location" = 8.0 (no trailing slash).
Click the green + in the first row 11 times.

Enter the following information in the new rows:

Type	Name	Dockerfile Location	Docker Tag Name
`Branch`	`master`	`dev-master`	`dev-master`
`Branch`	`master`	`8.0`	`8.0`
`Branch`	`master`	`7.1`	`7.1`
`Branch`	`master`	`7.0`	`7.0`
`Branch`	`master`	`6.6`	`6.6`
`Branch`	`master`	`6.5`	`6.5`
`Branch`	`master`	`6.4`	`6.4`
`Branch`	`master`	`6.3`	`6.3`
`Branch`	`master`	`6.2`	`6.2`
`Branch`	`master`	`6.1`	`6.1`
`Branch`	`master`	`6.0`	`6.0`

Folder and tag naming notes

There are a few options for naming folders / tags:

I could have named them the way Composer / the tags in drush-ops/drush / SemVer refer to them (i.e.: 6.0.0, 6.1.0, etc.)
I could have named them the way Drush refers to itself in drush --version (i.e.: 6.0, 6.1.0, etc.)
I could have used Drupal-style placeholders for the Patch version element (i.e.: 6.0.x, 6.1.x, etc.)
I could have left out the Patch version element entirely (i.e.: 6.0, 6.1, etc.).

I chose to leave out the Patch version element in my folder names and tag suggested names because:

I'm not aware of any maintainability impacts,
I suspect we don't want to add new tags / Dockerfiles for every patch at the current time,
SemVer doesn't define any standard placeholders, and,
Less typing

... however, I'm open to suggestions and happy to change it if necessary.

Special thanks to:

... for helping me understand how Composer interprets version numbers.