Add local dependencies check

Add checks for:

* Docker installed locally
* Docker buildx plugin installed locally
* Dockerfile exists

If checks fail, it will halt deployment and provide more specific error messages.

Also adds a cli subcommand:
`mrsk build dependencies`

Fixes: #109 and #237

lib/mrsk/commands/builder/base.rb

@@ -1,6 +1,9 @@
+
 class Mrsk::Commands::Builder::Base < Mrsk::Commands::Base
   delegate :argumentize, to: Mrsk::Utils
 
+  class BuilderError < StandardError; end
+
   def clean
     docker :image, :rm, "--force", config.absolute_image
   end
@@ -17,6 +20,7 @@ class Mrsk::Commands::Builder::Base < Mrsk::Commands::Base
     context
   end
 
+
   private
     def build_tags
       [ "-t", config.absolute_image, "-t", config.latest_image ]
@@ -35,7 +39,11 @@ class Mrsk::Commands::Builder::Base < Mrsk::Commands::Base
     end
 
     def build_dockerfile
-      argumentize "--file", dockerfile
+      if Pathname.new(File.expand_path(dockerfile)).exist?
+        argumentize "--file", dockerfile
+      else
+        raise BuilderError, "Missing Dockerfile"
+      end
     end
 
     def args