Initial provider implementation (#18)

This brings over the initial buildx prototype from pulumi/pulumi-docker
and fixes various build and release issues.

examples/main_test.go

@@ -0,0 +1,61 @@
+package examples
+
+import (
+	"crypto/rsa"
+	"errors"
+	"io"
+	"math/rand"
+	"net"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"golang.org/x/crypto/ssh/agent"
+)
+
+func TestMain(m *testing.M) {
+	sock := sshagent()
+	os.Setenv("SSH_AUTH_SOCK", sock)
+
+	os.Exit(m.Run())
+}
+
+// sshagent crates an in-memory SSH agent with one identity.
+func sshagent() string {
+	dir, err := os.MkdirTemp(os.TempDir(), "docker-test-*")
+	if err != nil {
+		panic(err)
+	}
+
+	sock := filepath.Join(dir, "test.sock")
+
+	l, err := net.Listen("unix", sock)
+	if err != nil {
+		panic(err)
+	}
+
+	a := agent.NewKeyring()
+	//nolint:gosec
+	key, err := rsa.GenerateKey(rand.New(rand.NewSource(42)), 2048)
+	if err != nil {
+		panic(err)
+	}
+	err = a.Add(agent.AddedKey{PrivateKey: key})
+	if err != nil {
+		panic(err)
+	}
+
+	go func() {
+		for {
+			conn, err := l.Accept()
+			if err != nil {
+				panic(err)
+			}
+			if err := agent.ServeAgent(a, conn); err != nil && !errors.Is(err, io.EOF) {
+				panic(err)
+			}
+		}
+	}()
+
+	return sock
+}