Docker Expert Guide

Container Runtime Detection

Detect container runtime from cgroup:

// Read /proc/<pid>/cgroup to detect container
func getContainerID(pid int) (string, string, error) {
    data, err := os.ReadFile(fmt.Sprintf("/proc/%d/cgroup", pid))
    if err != nil {
        return "", "", err
    }

    // Parse cgroup v1: "10:memory:/docker/<container-id>"
    // Parse cgroup v2: "0::/system.slice/docker-<container-id>.scope"
    for _, line := range strings.Split(string(data), "\n") {
        if strings.Contains(line, "docker") {
            // Extract container ID (64 hex chars)
        }
        if strings.Contains(line, "containerd") {
            // containerd runtime
        }
    }
    return "", "", nil
}

Supported runtimes: Docker, Podman, containerd, cri-o

Docker API via CLI

Prefer docker CLI over API for simplicity:

// Get container name
cmd := exec.Command("docker", "inspect", "--format", "{{.Name}}", containerID)
output, err := cmd.Output()

// List running containers
cmd := exec.Command("docker", "ps", "--format", "{{.ID}}\t{{.Names}}\t{{.Image}}")

// Execute in container
cmd := exec.Command("docker", "exec", containerID, "cat", "/proc/1/cmdline")

Docker Compose Detection

// Check for compose labels
cmd := exec.Command("docker", "inspect", "--format",
    "{{index .Config.Labels \"com.docker.compose.project\"}}", containerID)

// Get compose service name
cmd := exec.Command("docker", "inspect", "--format",
    "{{index .Config.Labels \"com.docker.compose.service\"}}", containerID)

Key labels:

•com.docker.compose.project - Project name
•com.docker.compose.service - Service name
•com.docker.compose.container-number - Replica number

Container Instrumentation Patterns

Volume Mount Approach

Mount agent JAR into container:

bash

docker run -v /opt/middleware/agents:/opt/agents:ro \
  -e JAVA_TOOL_OPTIONS="-javaagent:/opt/agents/middleware-javaagent.jar" \
  my-java-app

Docker Compose Override

yaml

# docker-compose.override.yml
services:
  my-service:
    volumes:
      - /opt/middleware/agents:/opt/agents:ro
    environment:
      - JAVA_TOOL_OPTIONS=-javaagent:/opt/agents/middleware-javaagent.jar

Container Networking

// Get container IP
cmd := exec.Command("docker", "inspect", "--format",
    "{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}", containerID)

// Get exposed ports
cmd := exec.Command("docker", "inspect", "--format",
    "{{range $p, $conf := .NetworkSettings.Ports}}{{$p}} {{end}}", containerID)

Debugging Containers

bash

# Enter running container
docker exec -it <container> /bin/sh

# View logs
docker logs -f <container>

# Check resource usage
docker stats <container>

# Inspect full config
docker inspect <container> | jq '.[0].Config'

# Check mounted volumes
docker inspect --format '{{range .Mounts}}{{.Source}} -> {{.Destination}}{{"\n"}}{{end}}' <container>

Common Issues

Permission Denied in Container

•Check :ro vs :rw volume mount
•Verify file ownership matches container user
•Use --user flag or set appropriate permissions

Container Not Found

•Container might have restarted (new ID)
•Use container name instead of ID when possible
•Implement retry with exponential backoff

Java Agent Not Loading

•Verify volume mount path exists in container
•Check JAVA_TOOL_OPTIONS is set correctly
•Ensure agent JAR is readable (chmod 644)

Go Docker Client Library

When CLI is insufficient, use official client:

import "github.com/docker/docker/client"

cli, err := client.NewClientWithOpts(client.FromEnv)
containers, err := cli.ContainerList(ctx, types.ContainerListOptions{})

Only use when needed - CLI is simpler for most operations.