docker build --force-rm=true should always remove intermediate containers

[sw00choi]

According to the documents listed below, docker build command with --force-rm=true option should always remove intermediate containers.

• https://apidocs.joyent.com/docker/commands/build
• https://docs.docker.com/engine/reference/commandline/build/

However, if I give option --rm=false --force-rm=true, sdc-docker does not remove the intermediate containers while the upstream docker removes them.

> docker build --rm=false --force-rm=true .
Sending build context to Docker daemon 2.048 kB
Step 1 : FROM ubuntu:latest
 ---> 56465e1e45d2
Step 2 : RUN touch /etc/hello
 ---> Running in 18042604d794
 ---> 327977df8d00
Step 3 : RUN touch /etc/world
 ---> Running in 18042604d794
 ---> c240401e32c0
Importing image 327977df8d00 into IMGAPI
Importing image c240401e32c0 into IMGAPI
Successfully built c240401e32c0

> docker ps -a
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS                      PORTS               NAMES
18042604d794        ubuntu              "/bin/sh -c 'touch /e"   30 seconds ago      Exited (0) 11 seconds ago                       build_ce53e610-b607-11e6-bbd0-6f73ca1fdff5

Well.. the combination of --rm=false and --force-rm=true might not be practical, but I want to fix the difference between the behaviors of sdc-docker and the upstream docker.

(I uploaded patch for this: https://cr.joyent.us/#/c/964/)

[twhiteman]

@sw00choi thanks for the fix