Commander needn't accumulate configuration

Commander had version/destination solely to incrementally accumulate CLI
options. Simpler to configure in one shot.

Clarifies responsibility and lets us introduce things like
`abbreviated_version` in one spot - Configuration.

lib/mrsk/configuration.rb

@@ -9,13 +9,12 @@ class Mrsk::Configuration
   delegate :service, :image, :servers, :env, :labels, :registry, :builder, to: :raw_config, allow_nil: true
   delegate :argumentize, :argumentize_env_with_secrets, to: Mrsk::Utils
 
-  attr_accessor :version
   attr_accessor :destination
   attr_accessor :raw_config
 
   class << self
-    def create_from(base_config_file, destination: nil, version: "missing")
-      raw_config = load_config_files(base_config_file, *destination_config_file(base_config_file, destination))
+    def create_from(config_file:, destination: nil, version: nil)
+      raw_config = load_config_files(config_file, *destination_config_file(config_file, destination))
 
       new raw_config, destination: destination, version: version
     end
@@ -38,14 +37,22 @@ class Mrsk::Configuration
       end
   end
 
-  def initialize(raw_config, destination: nil, version: "missing", validate: true)
+  def initialize(raw_config, destination: nil, version: nil, validate: true)
     @raw_config = ActiveSupport::InheritableOptions.new(raw_config)
     @destination = destination
-    @version = version
+    @declared_version = version
     valid? if validate
   end
 
 
+  def version=(version)
+    @declared_version = version
+  end
+
+  def version
+    @declared_version.presence || ENV["VERSION"] || current_commit_hash
+  end
+
   def abbreviated_version
     Mrsk::Utils.abbreviate_version(version)
   end
@@ -203,4 +210,13 @@ class Mrsk::Configuration
     def role_names
       raw_config.servers.is_a?(Array) ? [ "web" ] : raw_config.servers.keys.sort
     end
+
+    def current_commit_hash
+      @current_commit_hash ||=
+        if system("git rev-parse")
+          `git rev-parse HEAD`.strip
+        else
+          raise "Can't use commit hash as version, no git repository found in #{Dir.pwd}"
+        end
+    end
 end