fly io docs [skip ci]

doc/fly_io/Dockerfile

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+FROM babashka/babashka:0.8.2
+
+COPY example.clj /
+
+ENTRYPOINT bb /example.clj

doc/fly_io/README.md

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+# Deploying a babashka app to fly.io
+
+[Fly.io](https://fly.io/) is a service that can run full stack apps with minimal
+configuration. If you like the ease of Heroku, you might like fly.io and perhaps
+even better! This document shows how to get a minimal babashka application up
+and running on `fly.io`.
+
+In `example.clj` we start an http-kit web server which spits out some HTML. You
+can run this locally by invoking `bb example.clj` from the command line.
+
+To get this site running on `fly.io`, you need to
+[install](https://fly.io/docs/getting-started/installing-flyctl/) and [log
+in](https://fly.io/docs/getting-started/log-in-to-fly/).
+
+Then run `flyctl launch` to create a new application. After making changes, you
+can re-deploy the site with `flyctl deploy`.
+
+That's it!

doc/fly_io/example.clj

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+(ns example
+  (:require [hiccup2.core :refer [html]]
+            [org.httpkit.server :refer [run-server]]))
+
+(def port (or (some-> (System/getenv "PORT")
+                      parse-long)
+              8092))
+
+(run-server
+ (fn [_]
+   {:body
+    (str (html
+          [:html
+           [:body
+            [:h1 "Hello world!"]
+            [:p (str "This site is running with babashka v"
+                     (System/getProperty "babashka.version"))]]]))})
+ {:port port})
+
+(println "Site running on port" port)
+@(promise)

doc/fly_io/fly.toml

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+# fly.toml file generated for shy-sound-2847 on 2022-05-16T14:12:38+02:00
+
+app = "shy-sound-2847"
+
+kill_signal = "SIGINT"
+kill_timeout = 5
+processes = []
+
+[env]
+  PORT = "8092"
+
+[experimental]
+  allowed_public_ports = []
+  auto_rollback = true
+
+[[services]]
+  http_checks = []
+  internal_port = 8092
+  processes = ["app"]
+  protocol = "tcp"
+  script_checks = []
+
+  [services.concurrency]
+    hard_limit = 25
+    soft_limit = 20
+    type = "connections"
+
+  [[services.ports]]
+    force_https = true
+    handlers = ["http"]
+    port = 80
+
+  [[services.ports]]
+    handlers = ["tls", "http"]
+    port = 443
+
+  [[services.tcp_checks]]
+    grace_period = "1s"
+    interval = "15s"
+    restart_limit = 0
+    timeout = "2s"