Install nvm on Platform.sh

.environment

# Statements in this file will be executed (sourced) by the shell in SSH
# sessions, in deploy hooks, in cron jobs, and in the application's runtime
# environment.

# This loads nvm for general usage.
export NVM_DIR="$HOME/.nvm"
[ -s "$NVM_DIR/nvm.sh" ] && \. "$NVM_DIR/nvm.sh"

.platform.app.yaml

# The hooks executed at various points in the lifecycle of the application.
hooks:
    # The deploy hook runs after your application has been deployed and started.
    build: |
        unset NPM_CONFIG_PREFIX
        curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.33.8/install.sh | dash
        export NVM_DIR="$HOME/.nvm"
        [ -s "$NVM_DIR/nvm.sh" ] && \. "$NVM_DIR/nvm.sh"
        nvm current
        nvm install 9.5.0
    deploy: |
        unset NPM_CONFIG_PREFIX

Some related resources
https://docs.platform.sh/languages/nodejs/node-version.html
https://chromatichq.com/insights/one-nodejs-version-rule-them-all

You should probably ignore this and use what's at https://docs.platform.sh/languages/nodejs/node-version.html. This should work, but it was created before Platform.sh had better node version management.