Write the deployment guide and an example Caddy config (closes #89)

README.md gains a "Deployment" section: what the reverse proxy in front
of pixa must do (terminate TLS, pass Host, Origin and Referer on
unchanged, set X-Forwarded-For with trusted_proxies to match, wait at
least downstream_timeout, optionally refuse /metrics) and what pixa does
itself; that the state directory needs a persistent volume, what
cache_max_bytes counts and why to set it; the health check for a load
balancer; what SIGTERM does and the exit codes; and what running outside
Docker needs. configs/Caddyfile is the example, as Caddy needs no
settings beyond the host name and pixa's address.

Model: opus-5-5

README.md

@@ -34,6 +34,65 @@ else has a built-in default. A config file mounted at `/etc/pixa/config.yml`
 is optional: it is read when present, and an environment variable wins over
 the same setting in it.
 
+## Deployment
+
+pixa listens on plain HTTP and runs behind a reverse proxy that terminates TLS.
+[`configs/Caddyfile`](configs/Caddyfile) is an example for Caddy, chosen because
+it is the smallest correct one: Caddy gets the TLS certificate itself and does
+everything in this list without further settings. The reverse proxy must:
+
+- terminate TLS, as the login and generator pages work only over HTTPS (see
+  Routes);
+- pass the `Host`, `Origin` and `Referer` headers on unchanged, as pixa refuses
+  a form from those pages unless `Origin` or `Referer` names the host in `Host`,
+  and builds encrypted URLs from `Host`;
+- set `X-Forwarded-For` to the client's address, with `trusted_proxies` set to
+  the address pixa sees the proxy's requests come from, so the login limit
+  counts each client by its own address (see `trusted_proxies` under
+  Configuration);
+- wait for pixa's answer for at least `downstream_timeout` (default `60s`), the
+  longest pixa takes to fetch, convert and send an image.
+
+It may also refuse `/metrics`, as the example does, so that only a scraper that
+reaches pixa directly can read it; pixa itself asks for the metrics username and
+password there.
+
+pixa does the rest itself: it checks signatures and encrypted URLs, applies the
+allowlist, refuses upstream hosts with private or local addresses, limits login
+attempts, upstream response size and image dimensions, and sends the security
+headers, `Strict-Transport-Security` included, with every response.
+
+The state directory (`state_dir`, `/var/lib/pixa` in the container) holds the
+database and the disk cache:
+
+- It needs a persistent volume: without one, every restart starts with an empty
+  cache. In the container, the startup script gives the directory to the user
+  pixa runs as (uid 65532) and sets its mode to `750`; outside it, that user
+  must be able to write the directory.
+- `cache_max_bytes` limits the source and transformed images together. The
+  database, the metadata files, the `.meta` file beside each transformed image
+  and files still being written come on top, and eviction runs in the
+  background, so the cache can pass the limit for a while: leave room on the
+  volume beyond it.
+- Set `cache_max_bytes` for a lasting deployment. Its default is 75% of the
+  space free when pixa starts, which the cache's own files reduce, so a fuller
+  cache gives a smaller limit after a restart.
+
+A load balancer's health check can request `/.well-known/healthcheck.json`,
+which answers 200 whenever pixa is running, in maintenance mode too (see
+`maintenance_mode`).
+
+On SIGTERM or SIGINT pixa stops accepting connections, gives the requests in
+progress and the images being processed 5 seconds to finish, and exits: with 0,
+or with 1 when images were still being processed after those 5 seconds or
+another part of pixa failed to stop. A request not finished by then is cut off.
+`docker stop` waits 10 seconds before it kills the container.
+
+Outside Docker, pixa needs libvips (the image has 8.15) and libheif to run, as
+it uses libvips through CGO; building it also needs their development files,
+`pkg-config` and a C compiler. `script/bootstrap` installs all of these with
+nix, apt, brew or apk.
+
 ## Running under upaas
 
 What the [upaas](https://git.eeqj.de/sneak/upaas) app for pixa needs: