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.rb

@@ -33,4 +33,28 @@ class Mrsk::Commands::Builder < Mrsk::Commands::Base
   def multiarch_remote
     @multiarch_remote ||= Mrsk::Commands::Builder::Multiarch::Remote.new(config)
   end
+
+  def native_and_local?
+    name == 'native'
+  end
+
+  def dependencies
+    if native_and_local?
+      docker_version
+    else
+    combine \
+      docker_version,
+      docker_buildx_version
+    end
+  end
+
+  private
+
+    def docker_version
+      docker "--version"
+    end
+
+    def docker_buildx_version
+      docker :buildx, "version"
+    end
 end

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