Isso pode ser bobo, mas não consigo instalarcPickle com imagem do docker python 3.5

Dockerfile

FROM python:3.5-onbuild

requirements.txt

cpickle

Quando tento construir a imagem

$ docker build -t sample .
Sending build context to Docker daemon 3.072 kB
Step 1 : FROM python:3.5-onbuild
# Executing 3 build trigge,rs...
Step 1 : COPY requirements.txt /usr/src/app/
Step 1 : RUN pip install --no-cache-dir -r requirements.txt
 ---> Running in 016c35a032ee
Collecting cpickle (from -r requirements.txt (line 1))
  Could not find a version that satisfies the requirement cpickle (from -r requirements.txt (line 1)) (from versions: )
No matching distribution found for cpickle (from -r requirements.txt (line 1))
You are using pip version 7.1.2, however version 8.1.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
The command '/bin/sh -c pip install --no-cache-dir -r requirements.txt' returned a non-zero code: 1