docker build is very slow even with simple commands

I'm building a docker image on my Raspberry Pi, which is of course takes some time. The problem here is that even very simple commands in the Dockerfile like setting an environment variable, using chmod +x on a single file or exposing port 80 take minutes to complete.

Here is an excerpt of my Dockerfile:

FROM resin/rpi-raspbian
MAINTAINER felixbr <[email protected]>

RUN export DEBIAN_FRONTEND=noninteractive && apt-get update && apt-get install -y python python-dev python-pip python-numpy python-scipy python-mysqldb mysql-server redis-server nginx dos2unix poppler-utils

COPY requirements.txt /app/

RUN pip install -r /app/requirements.txt

COPY . /app

WORKDIR /app

RUN cp /app/nginx-django.cfg /etc/nginx/sites-enabled/default
RUN chmod +x /app/start.sh

ENV DOCKERIZED="true"

CMD ./start.sh

EXPOSE 80

Keep in mind this is using an ARMv6 base image, so it can run on a Raspberry Pi and I'm using docker 1.5.0 built for the hypriot Raspberry Pi OS.

Is it copying the built layers for every command or why does each of the last few commands take minutes to complete?

Solution

Each instruction of the Dockerfile will be run in a container. What it means is that for each instruction it will do the following :

Instantiate a container from the image created by the previous step, which will create a new layer (the R/W one)
Do the thing (pip install, etc..)
Commit, which will copy the top layer as an image layer (I'm pretty sure it is copying the layer)
And removing the container (if the --rm option is specified) (thus, removing the container Read/Write layer)

There are a few I/O operations involved. On an SSD it's really quick, as well as on a good hard drive. When you build it on the Raspberry PI, if you build it on the SD Card (or MicroSD), the performance of the SD card is probably not that good. It will depend on the class of you MicroSD and even then, I don't think it's really good for the card. I made the try with a simple node project, and it definitely took a few minutes instead of a few seconds like it did on my laptop. It is hardware related (mostly I/O for the SD Card, maybe a little bit the CPU, but...).

You might wanna try to use an external hard drive connected to the raspberry Pi and move the docker folders there, to see if the performance are better.