Docker run giving different result to docker build (trying to use 32bit image on 64bit host)

I’m trying to use a 32-bit docker image (from https://registry.hub.docker.com/u/protomouse/debian-i386/) on my 64-bit machine and am getting a result I wasn’t expecting.

First up, I’m using boot2docker and have v1.5.0 of the boot2docker image (the current version). My machine is Darwin x86_64.

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  • Once docker is all set up:

    $> docker pull protomouse/debian-i386:wheezy
    $> docker run protomouse/debian-i386:wheezy uname -a
    Linux c16621f70b0d 3.18.5-tinycore64 #1 SMP Sun Feb 1 06:02:30 UTC 2015 i686 GNU/Linux
    

    The i686 is looking very promising in the result from uname -a
    But when I go to making a dockerfile to extend the image, I get a different result. My dockerfile:

    $> cat Dockerfile
    FROM protomouse/debian-i386:wheezy
    
    RUN uname -a
    

    I build this with:

    $> docker build  --no-cache . 
    Sending build context to Docker daemon 3.032 MB
    Sending build context to Docker daemon 
    Step 0 : FROM protomouse/debian-i386:wheezy
     ---> 819840964f5f
    Step 1 : RUN uname -a
     ---> Running in defe4361f7db
    Linux 5eda7c5eb292 3.18.5-tinycore64 #1 SMP Sun Feb 1 06:02:30 UTC 2015 x86_64 GNU/Linux
     ---> 65df78215ee1
    Removing intermediate container defe4361f7db
    Successfully built 65df78215ee1
    

    Notice that the result of uname -a is now telling me I have x86_64.

    I’m fairly new to docker, so suspect I have fundamentally mis-understood the difference between the two operations, but I’d really appreciate it if anybody could shed any light on the matter.

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