Inspecting Containers and Images

Imagine an X-ray machine at an airport security checkpoint.

On the surface, a suitcase looks normal. But the X-ray reveals what’s inside: structure, contents, hidden compartments.

docker inspect is the X-ray for containers and images.

It reveals everything: configuration, state, networking, mounts, metadata.

Inspect a container:

docker inspect <container-name>

Returns JSON with full details.

Inspect an image:

docker image inspect <image-name>

Returns JSON with image metadata, layers, size, etc.

Extract specific fields with —format:

# Get container IP
docker inspect myapp --format='{{.NetworkSettings.IPAddress}}'

# Get exit code
docker inspect myapp --format='{{.State.ExitCode}}'

# Get mounts
docker inspect myapp --format='{{.Mounts}}'

# Get environment variables
docker inspect myapp --format='{{.Config.Env}}'

Container state:

# Is it running?
docker inspect myapp --format='{{.State.Running}}'

# Exit code
docker inspect myapp --format='{{.State.ExitCode}}'

# Started at
docker inspect myapp --format='{{.State.StartedAt}}'

# Finished at
docker inspect myapp --format='{{.State.FinishedAt}}'

Networking:

# IP address
docker inspect myapp --format='{{.NetworkSettings.IPAddress}}'

# Ports
docker inspect myapp --format='{{.NetworkSettings.Ports}}'

# Networks
docker inspect myapp --format='{{.NetworkSettings.Networks}}'

Volumes and mounts:

# All mounts
docker inspect myapp --format='{{json .Mounts}}' | jq

# Specific mount
docker inspect myapp --format='{{range .Mounts}}{{.Source}} -> {{.Destination}}{{"\n"}}{{end}}'

Environment variables:

docker inspect myapp --format='{{range .Config.Env}}{{println .}}{{end}}'

Image layers:

docker image inspect nginx:alpine --format='{{json .RootFS.Layers}}' | jq

Shows the SHA256 hashes of all layers in the image.

Image size:

docker image inspect nginx --format='{{.Size}}'

View layer history:

docker image history <image>

Shows each layer, the command that created it, and its size.

Example:

IMAGE          CREATED BY                                      SIZE
abc123         CMD ["nginx"]                                   0B
def456         COPY nginx.conf /etc/nginx/nginx.conf          2.5kB
ghi789         RUN apt-get install nginx                       45MB
jkl012         FROM ubuntu:22.04                               77MB

This shows how the image was built, layer by layer.

1. Debugging network issues:

# Find container IP
docker inspect web --format='{{.NetworkSettings.IPAddress}}'

# Check what networks it's on
docker inspect web --format='{{range $net, $config := .NetworkSettings.Networks}}{{$net}}: {{$config.IPAddress}}{{"\n"}}{{end}}'

2. Finding mounted volumes:

docker inspect db --format='{{range .Mounts}}{{.Name}}: {{.Source}} -> {{.Destination}}{{"\n"}}{{end}}'

3. Checking resource limits:

# Memory limit
docker inspect myapp --format='{{.HostConfig.Memory}}'

# CPU limit
docker inspect myapp --format='{{.HostConfig.CpuQuota}}'

4. Finding why a container exited:

# Exit code
docker inspect myapp --format='{{.State.ExitCode}}'

# OOM killed?
docker inspect myapp --format='{{.State.OOMKilled}}'

# Error message
docker inspect myapp --format='{{.State.Error}}'

5. Inspecting image metadata:

# When was it built?
docker image inspect myapp --format='{{.Created}}'

# What's the entrypoint?
docker image inspect myapp --format='{{.Config.Entrypoint}}'

# What's the CMD?
docker image inspect myapp --format='{{.Config.Cmd}}'

# Environment variables baked into the image
docker image inspect myapp --format='{{.Config.Env}}'

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