Fixes #9085: Cannot `up` a machine with Docker Toolbox and Git Bash

plugins/providers/docker/driver.rb

@@ -49,6 +49,13 @@ module VagrantPlugins
         run_cmd += volumes.map { |v|
           v = v.to_s
           if v.include?(":") && @executor.windows?
+            if v.index(":") != v.rindex(":")
+              # If we have 2 colons, the host path is an absolute Windows URL
+              # and we need to remove the colon from it
+              host, colon, guest = v.rpartition(":")
+              host = "//" + host[0].downcase + host[2..-1]
+              v = [host, guest].join(":")
+            else 
               host, guest = v.split(":", 2)
                   host = Vagrant::Util::Platform.windows_path(host)
                   # NOTE: Docker does not support UNC style paths (which also
@@ -57,6 +64,8 @@ module VagrantPlugins
                   host.gsub!(/^[^A-Za-z]+/, "")
                   v = [host, guest].join(":")
             end
+          end
+
           ['-v', v.to_s]
         }
         run_cmd += %W(--privileged) if params[:privileged]