Add local dependencies check

Add checks for:

* Docker installed locally
* Docker buildx plugin installed locally (not currently required for
  native/local build)
* Dockerfile exists

Checks will halt deployment and provide more specific error messages.

Also adds a cli subcommand:
`mrsk build dependencies`

Fixes: basecamp#109 and basecamp#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 @@ def deliver
   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 @@ def details
       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 @@ def multiarch
   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 @@ def build_context
     context
   end
 
+
   private
     def build_tags
       [ "-t", config.absolute_image, "-t", config.latest_image ]
@@ -35,7 +39,11 @@ def build_secrets
     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

test/cli/build_test.rb

@@ -9,13 +9,15 @@ 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
   end
 
   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 \