Permission error when running `gem install sinatra`

[npty]

Reproduce steps:

1. Clone the repo git clone [e-mail]:jpetazzo/container.training.git (commit 2e4e6bc as of now)
2. Go inside dockercoin folder cd dockercoins
3. Run docker-compose up

Run into the following issue

docker-compose up
Building hasher
Step 1/7 : FROM ruby:alpine
 ---> 9d6c19e9b063
Step 2/7 : RUN apk add --update build-base curl
 ---> Using cache
 ---> cb670ccae9bb
Step 3/7 : RUN gem install sinatra
 ---> Running in c6deff28dcf2
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /usr/local/bundle directory.
ERROR: Service 'hasher' failed to build: The command '/bin/sh -c gem install sinatra' returned a non-zero code: 1

[npty]

Found solution:
https://stackoverflow.com/questions/68243042/you-dont-have-write-permissions-for-the-usr-lib-ruby-gems-2-7-0-directory-alp

[Jérôme Petazzoni]

Interesting! Thank you for reporting the issue, and for following up with the fix. This can definitely be useful for others! 👍