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/cli/build.rb

@@ -1,4 +1,7 @@
 class Mrsk::Cli::Build < Mrsk::Cli::Base
+
+  class BuildError < StandardError; end
+
   desc "deliver", "Build app and push app image to registry then pull image on servers"
   def deliver
     with_lock do
@@ -10,16 +13,18 @@ class Mrsk::Cli::Build < Mrsk::Cli::Base
   desc "push", "Build and push app image to registry"
   def push
     with_lock do
-      cli = self
+      cli_build = self
 
       run_locally do
         begin
-          MRSK.with_verbosity(:debug) { execute *MRSK.builder.push }
+          if cli_build.dependencies
+            MRSK.with_verbosity(:debug) { execute *MRSK.builder.push }
+          end
         rescue SSHKit::Command::Failed => e
           if e.message =~ /(no builder)|(no such file or directory)/
             error "Missing compatible builder, so creating a new one first"
 
-            if cli.create
+            if cli_build.create
               MRSK.with_verbosity(:debug) { execute *MRSK.builder.push }
             end
           else
@@ -77,4 +82,19 @@ class Mrsk::Cli::Build < Mrsk::Cli::Base
       puts capture(*MRSK.builder.info)
     end
   end
+
+  desc "dependencies", "Check local dependencies"
+  def dependencies
+    run_locally do
+      begin
+        execute *MRSK.builder.dependencies
+      rescue SSHKit::Command::Failed => e
+        build_error = e.message =~ /command not found/ ?
+          "Docker is not installed locally" :
+          "Docker buildx plugin is not installed locally"
+        raise BuildError, build_error
+      end
+    end
+    true
+  end
 end

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