Ruby on Previewops

Previewops builds whatever your Dockerfile produces and runs it as a container, so Ruby needs no special support. What it does need is what every stack needs: read the injected PORT and listen on 0.0.0.0.


Read the port	`ENV["PORT"]`
Bind all interfaces	`puma -b tcp://0.0.0.0:$PORT`

Dockerfile

FROM ruby:3.3-slim
WORKDIR /app
COPY Gemfile Gemfile.lock ./
RUN bundle install
COPY . .
ENV PORT=8080
CMD ["sh", "-c", "bundle exec puma -b tcp://0.0.0.0:$PORT"]

ENV PORT=8080 is only a default. Previewops overrides it at deploy time, so the app must read the value rather than assume it.

Application code

A typical setup with Rails or Rack behind Puma:

# config/puma.rb
port ENV.fetch("PORT") { 8080 }

# Bind every interface, not just loopback.
bind "tcp://0.0.0.0:#{ENV.fetch('PORT') { 8080 }}"

environment ENV.fetch("RAILS_ENV") { "production" }

Ruby-specific pitfalls

config.force_ssl = true breaks readiness checks. Behind a proxy that terminates TLS it redirects the health check in a loop. Set config.assume_ssl = true as well, or disable force_ssl for previews.
RAILS_MASTER_KEY is required to boot a production Rails app. Store it as an env secret rather than baking it into the image.
Precompile assets at build time (RUN bundle exec rails assets:precompile), never in the start command — see the migration warning below.

Configuration

If your Dockerfile is not at the repository root, point at it from .previewops.yaml:

provider: cloud-run
dockerfile: ./docker/Dockerfile
port: 8080
ttl_hours: 24

See configuration.md for every available option.

Do not gate startup behind a migration

Language-agnostic, and the hardest failure here to diagnose:

# Avoid this
CMD ["sh", "-c", "run-migrations && start-server"]

If the migration cannot reach the database — a paused instance, an expired connection string, a cold-start timeout — it never exits, the server never starts, and the port never opens. The deploy itself already succeeded, so the preview shows as Live while nothing answers. Run migrations as a separate, visible step instead.

getting-started.md — your first deploy end to end
configuration.md — .previewops.yaml reference
databases.md — connecting a database to a preview
ui-deploy.md — deploying from the dashboard