Can't build current control image

There's #14 (closed), so I tried to rebuild the current control image, but I fail:

 ~/greenhost/bootstrap | docker build -t test control/
Sending build context to Docker daemon  32.05MB
Step 1/14 : FROM linuxbrew/linuxbrew
latest: Pulling from linuxbrew/linuxbrew
b849b56b69e7: Pull complete 
42986ef25bcd: Pull complete 
d927c1b717ec: Pull complete 
15b86ea20233: Pull complete 
e3f57d457970: Pull complete 
a780296c6c24: Pull complete 
64c5ba2baaa6: Pull complete 
2b2cdca2e536: Pull complete 
7a1e593081f2: Pull complete 
Digest: sha256:adea07cdcf18b97c5c4d5e3990cdc6cb7a83fb72ed805d413c649f7e5ecbfbaf
Status: Downloaded newer image for linuxbrew/linuxbrew:latest
 ---> 746a623d16a5
Step 2/14 : MAINTAINER arie@greenhost.nl
 ---> Running in 15af27560724
Removing intermediate container 15af27560724
 ---> c5da3139e713
Step 3/14 : RUN brew update
 ---> Running in cb2090fdeff6
==> Homebrew has enabled anonymous aggregate formulae and cask analytics.
Read the analytics documentation (and how to opt-out) here:
  https://docs.brew.sh/Analytics

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Updated 1 tap (homebrew/core).
==> Updated Formulae
angle-grinder
argus-clients
asciinema
bento4
bitwarden-cli
carla
ccache
chronograf
cmake
convox
dartsim
elixir
emscripten
fonttools
gnupg
gnuradio
gr-osmosdr
grafana
gtk+3
hexyl
jenkins
kube-aws
kubernetes-helm
lmod
lua
lua@5.1
maxwell
nomad
numpy
paket
pango
pdns
phpunit
pulumi
purescript
readline
squashfs
tcl-tk
teleport
tokei
uhd
vim
wxmaxima
Removing intermediate container cb2090fdeff6
 ---> fcb72320b70d
Step 4/14 : ADD brew/kubernetes-cli.rb /brew/kubernetes-cli.rb

 ---> 35c490f378eb
Step 5/14 : RUN brew install /brew/kubernetes-cli.rb
 ---> Running in 9e4b4ecd800b



Error: Permission denied @ rb_sysopen - /brew/kubernetes-cli.rb
The command '/bin/sh -c brew install /brew/kubernetes-cli.rb' returned a non-zero code: 1

Anyway, I'd like to move to a smaller docker image (#3 (closed)), but this also has it's issues. @arie, can we pair next week (or even Fr) on this ?