Below is a minimal, production-ready way to serve a SPA on Fly.io using Caddy ( It supports SPA history routing via a fallback file )

works with any applications that build to /dist

1) Dockerfile (multi-stage)

# syntax = docker/dockerfile:1

# Adjust NODE_VERSION as desired
ARG NODE_VERSION=22.17.0
FROM node:${NODE_VERSION}-slim as base

LABEL fly_launch_runtime="NodeJS"

# NodeJS app lives here
WORKDIR /app

# Set production environment
ENV NODE_ENV=production

# Throw-away build stage to reduce size of final image
FROM base as build

# Install packages needed to build node modules
RUN apt-get update -qq && \
    apt-get install -y python-is-python3 pkg-config build-essential 

# Install node modules
COPY --link package.json .
RUN npm install --production=false

# Copy application code
COPY --link . .

# Build application
RUN npm run build

# Remove development dependencies
RUN npm prune --production


FROM caddy:2.5.2-alpine

RUN apk add bash

COPY Caddyfile /etc/caddy/Caddyfile
RUN caddy fmt /etc/caddy/Caddyfile --overwrite

COPY --from=build /app/dist  /usr/share/caddy

# Expose port
EXPOSE 8043

# Start with goStatic and fallback to index.html for SPA routing
# CMD ["goStatic", "--fallback", "/srv/http/index.html"]

CMD ["caddy", "start", "--config", "/etc/caddy/Caddyfile", "--adapter", "caddyfile"]

Key bits: files go to /srv/http, default port is 8043, and -fallback /index.html handles SPA routes. 

2) fly.toml

After fly launch (choose “Dockerfile”), adjust fly.toml so Fly forwards public 80/443 to internal 8043:

app = "your-spa-app"
primary_region = "syd"
[build]
   # Uses the Dockerfile in the repo
  [[services]]
    internal_port = 8043
    processes = ["app"]
  [[services.ports]]
    handlers = ["http"]
    port = 80
  [[services.ports]]
    andlers = ["tls", "http"]
    port = 443
  # Optional: health check hits goStatic’s /health when enabled
  [services.concurrency]
    type = "connections"
    hard_limit = 200
    soft_limit = 100
  [[services.http_checks]]
    interval = "10s"
    timeout = "2s"
    method = "GET"
    path = "/health"
    protocol = "http"
    grace_period = "20s"

5) Caddyfile

:8043 {
   handle {
        root * /usr/share/caddy
        # requests with Accept-Encoding get automatically gzipped
        encode gzip
        # try to serve the file, if it doesn't exist, then serve index.html
        try_files {path} /index.html
        file_server
   }
}

4) Deploy

# one-time

fly auth signup   # or fly auth login

fly launch        # accept Dockerfile; pick region; don’t create a DB

# then

fly deploy

fly open