testcontainers: add tcp echo server container

testcontainers/echoserver/Dockerfile

@@ -0,0 +1,7 @@
+FROM golang:1.21
+
+WORKDIR /echoserver
+COPY echoserver.go .
+RUN go build -o /bin/echoserver echoserver.go
+
+ENTRYPOINT [ "/bin/echoserver" ]

testcontainers/echoserver/echoserver.go

@@ -0,0 +1,57 @@
+package main
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"strings"
+)
+
+func main() {
+	lAddr := &net.TCPAddr{
+		Port: 6666,
+	}
+
+	srv, err := net.ListenTCP("tcp4", lAddr)
+	if err != nil {
+		log.Fatalf("setting up listener: %v", err)
+	}
+
+	log.Printf("Listening on %s", lAddr)
+
+	for {
+		conn, err := srv.Accept()
+		if err != nil {
+			log.Fatalf("accepting connection: %v", err)
+		}
+
+		go func() {
+			log.Printf("received connection from %s", conn.RemoteAddr())
+			err := echo(conn)
+			if errors.Is(err, io.EOF) {
+				log.Printf("%s closed the connection", conn.RemoteAddr())
+			} else if err != nil {
+				log.Printf("%s: %v", conn.RemoteAddr(), err)
+			}
+		}()
+	}
+}
+
+func echo(conn net.Conn) error {
+	lineReader := bufio.NewReader(conn)
+	for {
+		line, err := lineReader.ReadString('\n')
+		if err != nil {
+			return fmt.Errorf("reading from peer: %w", err)
+		}
+
+		log.Printf("%s: %s", conn.RemoteAddr(), strings.TrimSpace(line))
+		_, err = conn.Write([]byte(line))
+		if err != nil {
+			return fmt.Errorf("echoing back to peer: %w", err)
+		}
+	}
+}