Reaper doesn't appear to be thread safe

[nwv-danvulpe]

Describe the bug
Having recently upgraded from 0.4.0 to 0.5.1 I noticed some tests started failing with:

could not start container: failed to create container: connecting to reaper failed: Connecting to Ryuk on  failed: dial tcp: missing address

We run a number of tests in parallel (in the same package) spinning up a few containers (less than 5). If we remove t.Parallel() the test pass successfully, but they take longer to complete.

To Reproduce
Can you write a unit test to reproduce your issue?

import (
	"context"
	"fmt"
	"github.com/testcontainers/testcontainers-go"
	"github.com/testcontainers/testcontainers-go/wait"
	"testing"
	"time"
)

func TestParallelContainers(t *testing.T) {
	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Minute)
	t.Cleanup(cancel)

	for i := 0; i < 3; i++ {
		t.Run(fmt.Sprintf("iteration_%d", i), func(t *testing.T) {
			t.Parallel()
			createTestContainer(t, ctx)
		})
	}
}

func createTestContainer(t *testing.T, ctx context.Context) int {
	req := testcontainers.ContainerRequest{
		Image:        "localstack/localstack:latest",
		ExposedPorts: []string{"4584/tcp", "8080/tcp"},
		Env: map[string]string{
			"SERVICES": "secretsmanager",
		},
		WaitingFor: wait.ForListeningPort("4584/tcp"),
		//SkipReaper: true,
	}
	container, err := testcontainers.GenericContainer(ctx, testcontainers.GenericContainerRequest{
		ContainerRequest: req,
		Started:          true,
	})
	if err != nil {
		t.Fatalf("could not start container: %v", err)
	}
	port, err := container.MappedPort(ctx, "4584")
	if err != nil {
		t.Fatalf("could not get mapped port: %v", err)
	}

	t.Cleanup(func() {
		container.Terminate(context.Background())
	})

	return port.Int()
}

Expected behavior
It should be possible to start containers in parallel.

Additional context
Test output:

=== RUN   TestParallelContainers
--- FAIL: TestParallelContainers (0.00s)
=== RUN   TestParallelContainers/iteration_0
=== PAUSE TestParallelContainers/iteration_0
=== CONT  TestParallelContainers/iteration_0
    --- FAIL: TestParallelContainers/iteration_0 (0.01s)
=== RUN   TestParallelContainers/iteration_1
=== PAUSE TestParallelContainers/iteration_1
=== CONT  TestParallelContainers/iteration_1
    TestParallelContainers/iteration_1: demo_test.go:39: could not start container: failed to create container: connecting to reaper failed: Connecting to Ryuk on  failed: dial tcp: missing address
    TestParallelContainers/iteration_0: demo_test.go:39: could not start container: failed to create container: connecting to reaper failed: Connecting to Ryuk on  failed: dial tcp: missing address
2020/05/12 09:57:27 Starting container id: 87b3a8c0ba74 image: quay.io/testcontainers/ryuk:0.2.3
2020/05/12 09:57:28 Waiting for container id 87b3a8c0ba74 image: quay.io/testcontainers/ryuk:0.2.3
2020/05/12 09:57:28 Container is ready id: 87b3a8c0ba74 image: quay.io/testcontainers/ryuk:0.2.3
2020/05/12 09:57:28 Starting container id: c3bf40970170 image: localstack/localstack:latest
2020/05/12 09:57:28 Waiting for container id c3bf40970170 image: localstack/localstack:latest
2020/05/12 09:57:43 Container is ready id: c3bf40970170 image: localstack/localstack:latest
    --- FAIL: TestParallelContainers/iteration_1 (0.01s)
=== RUN   TestParallelContainers/iteration_2
=== PAUSE TestParallelContainers/iteration_2
=== CONT  TestParallelContainers/iteration_2
    --- PASS: TestParallelContainers/iteration_2 (16.15s)
FAIL

Process finished with exit code 1

[gianarb]

I think something broke when working at #174 .
I will have a look at thanks a lot for writing such a nice reproducible gem!