Check that we have valid contexts before building

Load the hosts from the contexts before trying to build.

If there is no context, we'll create one. If there is one but the hosts
don't match we'll re-create.

Where we just have a local context, there won't be any hosts but we
still inspect the builder to check that it exists.

test/cli/build_test.rb

@@ -21,6 +21,10 @@ class CliBuildTest < CliTestCase
         .with(:git, "-C", anything, :status, "--porcelain")
         .returns("")
 
+      SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
+        .with(:docker, :buildx, :inspect, "kamal-app-multiarch", "> /dev/null")
+        .returns("")
+
       run_command("push", "--verbose").tap do |output|
         assert_hook_ran "pre-build", output, **hook_variables
         assert_match /Cloning repo into build directory/, output
@@ -121,11 +125,9 @@ class CliBuildTest < CliTestCase
       SSHKit::Backend::Abstract.any_instance.expects(:execute)
         .with(:docker, :buildx, :create, "--use", "--name", "kamal-app-multiarch")
 
-      SSHKit::Backend::Abstract.any_instance.expects(:execute)
-        .with { |*args| args[0..1] == [ :docker, :buildx ] }
+      SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
+        .with(:docker, :buildx, :inspect, "kamal-app-multiarch", "> /dev/null")
         .raises(SSHKit::Command::Failed.new("no builder"))
-        .then
-        .returns(true)
 
       SSHKit::Backend::Abstract.any_instance.expects(:execute).with { |*args| args.first.start_with?("git") }
 
@@ -137,6 +139,9 @@ class CliBuildTest < CliTestCase
         .with(:git, "-C", anything, :status, "--porcelain")
         .returns("")
 
+      SSHKit::Backend::Abstract.any_instance.expects(:execute)
+        .with(:docker, :buildx, :build, "--push", "--platform", "linux/amd64,linux/arm64", "--builder", "kamal-app-multiarch", "-t", "dhh/app:999", "-t", "dhh/app:latest", "--label", "service=\"app\"", "--file", "Dockerfile", ".")
+
       run_command("push").tap do |output|
         assert_match /WARN Missing compatible builder, so creating a new one first/, output
       end

test/cli/cli_test_case.rb

@@ -36,6 +36,9 @@ class CliTestCase < ActiveSupport::TestCase
         .with { |arg1, arg2| arg1 == :mkdir && arg2 == ".kamal/locks/app" }
       SSHKit::Backend::Abstract.any_instance.stubs(:execute)
         .with { |arg1, arg2| arg1 == :rm && arg2 == ".kamal/locks/app/details" }
+      SSHKit::Backend::Abstract.any_instance.stubs(:capture_with_info)
+        .with { |*args| args[0..2] == [ :docker, :buildx, :inspect ] }
+        .returns("")
     end
 
     def assert_hook_ran(hook, output, version:, service_version:, hosts:, command:, subcommand: nil, runtime: false)

test/cli/main_test.rb

@@ -116,6 +116,10 @@ class CliMainTest < CliTestCase
       .with(:git, "-C", anything, :status, "--porcelain")
       .returns("")
 
+    SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
+      .with(:docker, :buildx, :inspect, "kamal-app-multiarch", "> /dev/null")
+      .returns("")
+
     assert_raises(Kamal::Cli::LockError) do
       run_command("deploy")
     end
@@ -145,6 +149,10 @@ class CliMainTest < CliTestCase
       .with(:git, "-C", anything, :status, "--porcelain")
       .returns("")
 
+    SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
+      .with(:docker, :buildx, :inspect, "kamal-app-multiarch", "> /dev/null")
+      .returns("")
+
     assert_raises(SSHKit::Runner::ExecuteError) do
       run_command("deploy")
     end