Update Dockerfile

1 job for master in 20 seconds (queued for 2 seconds)
Name Stage Failure
failed
build Build
 ---> Using cache
---> 212d2c0b0bbf
Step 8/9 : RUN rvm install 2.6.6 && rvm alias create 2.6 ruby-2.6.6 && rvm use --default 2.6.6
---> Running in 4aa5be1b1f97
/bin/sh: rvm: command not found
The command '/bin/sh -c rvm install 2.6.6 && rvm alias create 2.6 ruby-2.6.6 && rvm use --default 2.6.6' returned a non-zero code: 127
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 127