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

test/cli/build_test.rb

@@ -9,6 +9,7 @@ class CliBuildTest < CliTestCase
   end
 
   test "push" do
+    Mrsk::Cli::Build.any_instance.stubs(:dependencies).returns(true)
     run_command("push").tap do |output|
       assert_match /docker buildx build --push --platform linux\/amd64,linux\/arm64 --builder mrsk-app-multiarch -t dhh\/app:999 -t dhh\/app:latest --label service="app" --file Dockerfile \. as .*@localhost/, output
     end
@@ -16,6 +17,7 @@ class CliBuildTest < CliTestCase
 
   test "push without builder" do
     stub_locking
+    Mrsk::Cli::Build.any_instance.stubs(:dependencies).returns(true)
     SSHKit::Backend::Abstract.any_instance.stubs(:execute)
       .with { |arg| arg == :docker }
       .raises(SSHKit::Command::Failed.new("no builder"))
@@ -68,6 +70,22 @@ class CliBuildTest < CliTestCase
     end
   end
 
+  test "dependencies" do
+    Mrsk::Commands::Builder.any_instance.stubs(:native_and_local?).returns(false)
+    run_command("dependencies").tap do |output|
+      assert_match /docker --version && docker buildx version/, output
+    end
+  end
+
+  test "dependencies with no buildx plugin" do
+    SSHKit::Backend::Abstract.any_instance.stubs(:execute)
+      .with(:docker, "--version", "&&", :docker, :buildx, "version")
+      .raises(SSHKit::Command::Failed.new("no buildx"))
+
+    Mrsk::Commands::Builder.any_instance.stubs(:native_and_local?).returns(false)
+    assert_raises(Mrsk::Cli::Build::BuildError) { run_command("dependencies") }
+  end
+
   private
     def run_command(*command)
       stdouted { Mrsk::Cli::Build.start([*command, "-c", "test/fixtures/deploy_with_accessories.yml"]) }

test/commands/builder_test.rb

@@ -52,12 +52,21 @@ class CommandsBuilderTest < ActiveSupport::TestCase
   end
 
   test "build dockerfile" do
+    Pathname.any_instance.expects(:exist?).returns(true).once
     builder = new_builder_command(builder: { "dockerfile" => "Dockerfile.xyz" })
     assert_equal \
       "-t dhh/app:123 -t dhh/app:latest --label service=\"app\" --file Dockerfile.xyz",
       builder.target.build_options.join(" ")
   end
 
+  test "missing dockerfile" do
+    Pathname.any_instance.expects(:exist?).returns(false).once
+    builder = new_builder_command(builder: { "dockerfile" => "Dockerfile.xyz" })
+    assert_raises(Mrsk::Commands::Builder::Base::BuilderError) do
+      builder.target.build_options.join(" ")
+    end
+  end
+
   test "build context" do
     builder = new_builder_command(builder: { "context" => ".." })
     assert_equal \