Error installing bundler, from Dockerfile

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Error building Dockerfile on ruby >= 2.6.9.

Dockerfile:

FROM ruby:2.6.9-alpine
RUN gem install bundler:2.1.4
RUN bundle config set no-cache 'true'
RUN bundle install

ADD . .

outputs:

 ---> Running in e2bc34dbc146
ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /usr/local/bundle directory.
The command '/bin/sh -c gem install bundler:2.1.4' returned a non-zero code: 1

Circleci configured with base image cimg/ruby:2.6.9

ps: similar error Error with installing bundler, but that does not apply here, as the final solution is a docker-compose file with 5 dependencies/services.
pps: the same steps were working ruby:2.5.9-alpine and circleci/ruby:2.5.9