Custom Dockerfile

Reach for a Dockerfile when customization matters

Previewly can synthesize runnable images straight from manifests such as package.json or go.mod. When generated defaults miss a build stage, distroless base layer, pinned CA bundle, GPU-less compiler flags, secrets baked through build-kit mounts, or anything else that needs explicit Docker authoring, declare dockerfile alongside your path.

How Previewly resolves an image today

Previewly parses your repo-root config (previewly.toml / yaml / yml) and honours an explicit dockerfile path first.
If no config-supplied Dockerfile was requested, Previewly probes the server default (Dockerfile unless your operator customised it) beneath the archived repository root—useful for teams migrating incrementally.
If neither artefact exists, Previewly derives a Dockerfile from heuristic stack detection (runtime hints, lockfiles, startup_* fields) prior to invoking Fly.

Layouts and monorepos

Colocate Dockerfile context with Dockerfile content so COPY behaves identically locally and during preview builds.
Monorepo example: dockerfile = "apps/web/Dockerfile" while keeping install commands rooted to that subdirectory with multi-stage layering.
Optionally add .dockerignore beside the Dockerfile's build context root to trim node_modules artefacts, binaries, caches, secrets, etc.

Checklist authored containers must satisfy

Default process bootstraps the HTTP workload without prompting for interactive input.
Listening socket binds to 0.0.0.0 on the Previewly-internal port surfaced by Fly (Previewly merges PORT beforehand).
EXPOSE aligns with whichever port Previewly configures in its generated Fly app manifest (internal_port / server default).

Stack notes

Stack	Reminder
Next.js standalone	Multi-stage Dockerfile copies traced output from `.next/standalone` so you do not reinstall dev tooling in the runtime slice.
Python workers	Honour `$PORT` with Gunicorn, Uvicorn, or waitress flags so Fly health checks succeed promptly.
Rust / JVM / statically linked Go	Binary targets should read Fly's injected environment for host binding—Previewly forwards the upstream PORT contract unchanged.
Static assets	Pair nginx/Caddy—or rely on Previewly's generated static shim—listening on whichever internal port aligns with Previewly routing.

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