How many CPUs does a docker container use?

As Charles mentions, by default all can be used, or you can limit it per container using the --cpuset-cpus parameter.

docker run --cpuset-cpus="0-2" myapp:latest

That would restrict the container to 3 CPU's (0, 1, and 2). See the docker run docs for more details.

The preferred way to limit CPU usage of containers is with a fractional limit on CPUs:

docker run --cpus 2.5 myapp:latest

That would limit your container to 2.5 cores on the host.

Lastly, if you run docker inside of a VM, including Docker for Mac, Docker for Windows, and docker-machine, those VM's will have a CPU limit separate from your laptop itself. Docker runs inside of that VM and will use all the resources given to the VM itself. E.g. with Docker for Mac you have the following menu:

Maybe your host VM has only one core by default. Therefore you should increase your VM cpu-count first and then use --cpuset-cpus option to increase your docker cores. You can remove docker default VM using the following command then you can create another VM with optional cpu-count and memory size.:

docker-machine rm default
docker-machine create -d virtualbox --virtualbox-cpu-count=8 --virtualbox-memory=4096 --virtualbox-disk-size=50000 default

After this step you can specify number of cores before running your image. this command will use 4 cores of total 8 cores.

docker run -it --cpuset-cpus="0-3" your_image_name

Then you can check number of available core in your image using this command:

nproc