Packer vs Docker
Compare machine-image creation with container-image creation.
Architecture
Packer
Packer is an image automation tool used to build machine images such as AMIs or VM templates. It is commonly used in immutable infrastructure workflows for virtual machines.
Containers
Docker
Docker is a container platform used to build and run container images. It packages applications and dependencies into portable runtime units for containerized environments.
Key Differences
Packer builds machine images, while Docker builds container images.
Packer images are intended to boot as full virtual machines, while Docker images are intended to run as containers.
Packer fits immutable infrastructure at the VM level, while Docker fits immutable packaging at the container level.
Packer is often used in EC2 or VM-based delivery workflows, while Docker is used in containerized application delivery workflows.
Docker images are lighter and process-focused, while machine images contain much more complete operating system state.
These tools solve different packaging layers even though both produce reusable images.
When to Use
When to use Packer
Use Packer when your delivery model relies on pre-baked virtual machine images such as AMIs for consistent VM provisioning.
When to use Docker
Use Docker when your application is packaged and deployed as a container into Docker, ECS, Kubernetes, or similar platforms.
Tradeoffs
Packer is ideal for VM-based immutable infrastructure, but less relevant in purely container-first platforms.
Docker is ideal for containerized delivery, but not a replacement for machine images when full VM boot environments are required.
Both can coexist in the same organization because they solve different image-building layers.
Common Mistakes
Treating Packer and Docker as interchangeable because both produce images.
Using Packer where container packaging is the real requirement.
Using Docker where full machine boot images are the actual deployment unit.
Interview Tip
A clean short answer is: Packer builds machine images, Docker builds container images.