diff --git a/dockerimg/dockerimg.go b/dockerimg/dockerimg.go
index 481d24e..039c84d 100644
--- a/dockerimg/dockerimg.go
+++ b/dockerimg/dockerimg.go
@@ -4,6 +4,7 @@
 import (
 	"bytes"
 	"context"
+	"crypto/rand"
 	"crypto/sha256"
 	"encoding/hex"
 	"encoding/json"
@@ -72,7 +73,11 @@
 // It writes status to stdout.
 func LaunchContainer(ctx context.Context, stdout, stderr io.Writer, config ContainerConfig) error {
 	if _, err := exec.LookPath("docker"); err != nil {
-		return fmt.Errorf("cannot find `docker` binary; run: brew install docker colima && colima start")
+		if runtime.GOOS == "darwin" {
+			return fmt.Errorf("cannot find `docker` binary; run: brew install docker colima && colima start")
+		} else {
+			return fmt.Errorf("cannot find `docker` binary; install docker (e.g., apt-get install docker.io)")
+		}
 	}
 
 	if out, err := combinedOutput(ctx, "docker", "ps"); err != nil {
@@ -241,7 +246,7 @@
 		// the scrollback (which is not good, but also not fatal).  I can't see why it does this
 		// though, since none of the calls in postContainerInitConfig obviously write to stdout
 		// or stderr.
-		if err := postContainerInitConfig(ctx, localAddr, commit, gitSrv.gitPort); err != nil {
+		if err := postContainerInitConfig(ctx, localAddr, commit, gitSrv.gitPort, gitSrv.pass); err != nil {
 			slog.ErrorContext(ctx, "LaunchContainer.postContainerInitConfig", slog.String("err", err.Error()))
 			errCh <- appendInternalErr(err)
 		}
@@ -304,6 +309,7 @@
 	gitLn   net.Listener
 	gitPort string
 	srv     *http.Server
+	pass    string
 }
 
 func (gs *gitServer) shutdown(ctx context.Context) {
@@ -317,8 +323,18 @@
 	return gs.srv.Serve(gs.gitLn)
 }
 
+func mkRandToken() string {
+	var b [16]byte
+	if _, err := rand.Read(b[:]); err != nil {
+		panic(err)
+	}
+	return hex.EncodeToString(b[:])
+}
+
 func newGitServer(gitRoot string) (*gitServer, error) {
 	ret := &gitServer{}
+	ret.pass = mkRandToken()
+
 	gitLn, err := net.Listen("tcp4", ":0")
 	if err != nil {
 		return nil, fmt.Errorf("git listen: %w", err)
@@ -326,7 +342,7 @@
 	ret.gitLn = gitLn
 
 	srv := http.Server{
-		Handler: &gitHTTP{gitRepoRoot: gitRoot},
+		Handler: &gitHTTP{gitRepoRoot: gitRoot, pass: ret.pass},
 	}
 	ret.srv = &srv
 
@@ -357,6 +373,8 @@
 	if relPath != "." {
 		cmdArgs = append(cmdArgs, "-w", "/app/"+relPath)
 	}
+	// colima does this by default, but Linux docker seems to need this set explicitly
+	cmdArgs = append(cmdArgs, "--add-host", "host.docker.internal:host-gateway")
 	cmdArgs = append(
 		cmdArgs,
 		imgName,
@@ -415,9 +433,16 @@
 		slog.DebugContext(ctx, "go", slog.Duration("elapsed", time.Now().Sub(start)), slog.String("path", cmd.Path), slog.String("args", fmt.Sprintf("%v", skribe.Redact(cmd.Args))))
 	}
 
-	src := filepath.Join(linuxGopath, "bin", "linux_"+runtime.GOARCH, "sketch")
+	var src string
+	if runtime.GOOS != "linux" {
+		src = filepath.Join(linuxGopath, "bin", "linux_"+runtime.GOARCH, "sketch")
+	} else {
+		// If we are already on Linux, there's no extra platform name in the path
+		src = filepath.Join(linuxGopath, "bin", "sketch")
+	}
+
 	dst := filepath.Join(path, "tmp-sketch-binary-linux")
-	if err := os.Rename(src, dst); err != nil {
+	if err := moveFile(src, dst); err != nil {
 		return "", err
 	}
 
@@ -444,11 +469,11 @@
 }
 
 // Contact the container and configure it.
-func postContainerInitConfig(ctx context.Context, localAddr, commit, gitPort string) error {
+func postContainerInitConfig(ctx context.Context, localAddr, commit, gitPort string, gitPass string) error {
 	localURL := "http://" + localAddr
 	initMsg, err := json.Marshal(map[string]string{
 		"commit":          commit,
-		"git_remote_addr": "http://host.docker.internal:" + gitPort + "/.git",
+		"git_remote_addr": fmt.Sprintf("http://sketch:%s@host.docker.internal:%s/.git", gitPass, gitPort),
 		"host_addr":       localAddr,
 	})
 	if err != nil {
@@ -671,3 +696,40 @@
 		fmt.Fprintf(os.Stderr, "failed to open browser: %v: %s\n", err, b)
 	}
 }
+
+// moveFile is like Python's shutil.move, in that it tries a rename, and, if that fails,
+// copies and deletes
+func moveFile(src, dst string) error {
+	if err := os.Rename(src, dst); err == nil {
+		return nil
+	}
+
+	stat, err := os.Stat(src)
+	if err != nil {
+		return err
+	}
+
+	sourceFile, err := os.Open(src)
+	if err != nil {
+		return err
+	}
+	defer sourceFile.Close()
+
+	destFile, err := os.Create(dst)
+	if err != nil {
+		return err
+	}
+	defer destFile.Close()
+
+	_, err = io.Copy(destFile, sourceFile)
+	if err != nil {
+		return err
+	}
+
+	sourceFile.Close()
+	destFile.Close()
+
+	os.Chmod(dst, stat.Mode())
+
+	return os.Remove(src)
+}
diff --git a/dockerimg/githttp.go b/dockerimg/githttp.go
index d618ab6..6f0ec55 100644
--- a/dockerimg/githttp.go
+++ b/dockerimg/githttp.go
@@ -1,16 +1,19 @@
 package dockerimg
 
 import (
+	"crypto/subtle"
 	"fmt"
 	"log/slog"
 	"net/http"
 	"net/http/cgi"
 	"os/exec"
+	"runtime"
 	"strings"
 )
 
 type gitHTTP struct {
 	gitRepoRoot string
+	pass        string
 }
 
 func (g *gitHTTP) ServeHTTP(w http.ResponseWriter, r *http.Request) {
@@ -22,11 +25,40 @@
 			http.Error(w, fmt.Sprintf("panic: %v\n", err), http.StatusInternalServerError)
 		}
 	}()
-	if !strings.HasPrefix(r.RemoteAddr, "127.0.0.1:") {
-		slog.InfoContext(r.Context(), "githttp: denied", "remote addr", r.RemoteAddr)
-		http.Error(w, "no", http.StatusUnauthorized)
+
+	// Get the Authorization header
+	username, password, ok := r.BasicAuth()
+
+	// Check if credentials were provided
+	if !ok {
+		// No credentials provided, return 401 Unauthorized
+		w.Header().Set("WWW-Authenticate", `Basic realm="Sketch Git Repository"`)
+		http.Error(w, "Unauthorized", http.StatusUnauthorized)
+		slog.InfoContext(r.Context(), "githttp: denied (basic auth)", "remote addr", r.RemoteAddr)
 		return
 	}
+
+	// Perform constant-time comparison to prevent timing attacks
+	usernameMatch := subtle.ConstantTimeCompare([]byte(username), []byte("sketch")) == 1
+	passwordMatch := subtle.ConstantTimeCompare([]byte(password), []byte(g.pass)) == 1
+
+	// Check if credentials are valid
+	if !usernameMatch || !passwordMatch {
+		w.Header().Set("WWW-Authenticate", `Basic realm="Git Repository"`)
+		http.Error(w, "Unauthorized", http.StatusUnauthorized)
+		slog.InfoContext(r.Context(), "githttp: denied (basic auth)", "remote addr", r.RemoteAddr)
+		return
+	}
+
+	if runtime.GOOS == "darwin" {
+		// On the Mac, Docker connections show up from localhost. On Linux, the docker
+		// network is more arbitrary, so we don't do this additional check there.
+		if !strings.HasPrefix(r.RemoteAddr, "127.0.0.1:") {
+			slog.InfoContext(r.Context(), "githttp: denied", "remote addr", r.RemoteAddr)
+			http.Error(w, "no", http.StatusUnauthorized)
+			return
+		}
+	}
 	gitBin, err := exec.LookPath("git")
 	if err != nil {
 		http.Error(w, "no git: "+err.Error(), http.StatusInternalServerError)