Add customizable configuration script to release (#2295)

README.md

@@ -349,6 +349,11 @@ When clustering is enabled, you must additionally set the following env vars:
 `LIVEBOOK_DISTRIBUTION` is automatically set to `name` if clustering is
 enabled.
 
+Some variables, like `LIVEBOOK_NODE`, are oftentimes computed at runtime.
+When using the Livebook Docker image, you can create a file at `/app/env.sh`
+that exports the necessary environment variables. This file is invoked right
+before booting Livebook.
+
 ## Development
 
 Livebook is primarily a Phoenix web application and can be setup as such:

lib/livebook/hubs/dockerfile.ex

@@ -130,16 +130,12 @@ defmodule Livebook.Hubs.Dockerfile do
 
         """ <>
           ~S"""
-          # Custom startup script to cluster multiple Livebook nodes on Fly.io
+          # Runtime configuration to cluster multiple Livebook nodes on Fly.io
           RUN printf '\
-          #!/bin/bash\n\
           export ERL_AFLAGS="-proto_dist inet6_tcp"\n\
           export LIVEBOOK_NODE="${FLY_APP_NAME}-${FLY_IMAGE_REF##*-}@${FLY_PRIVATE_IP}"\n\
           export LIVEBOOK_CLUSTER="dns:${FLY_APP_NAME}.internal"\n\
-          /app/bin/livebook start\n\
-          ' > /app/bin/start.sh && chmod +x /app/bin/start.sh
-
-          CMD [ "sh", "-c", "/app/bin/start.sh" ]
+          ' > /app/env.sh
           """
       end
 

rel/server/env.bat.eex

@@ -1,3 +1,7 @@
+if exist "!RELEASE_ROOT!\env.bat" (
+  call "!RELEASE_ROOT!\env.bat"
+)
+
 set RELEASE_MODE=interactive
 if defined LIVEBOOK_NODE set RELEASE_NODE="!LIVEBOOK_NODE!"
 if not defined RELEASE_NODE set RELEASE_NODE=livebook_server

rel/server/env.sh.eex

@@ -1,3 +1,7 @@
+if [ -f "${RELEASE_ROOT}/env.sh" ]; then
+  . "${RELEASE_ROOT}/env.sh"
+fi
+
 export RELEASE_MODE=interactive
 export RELEASE_NODE=${LIVEBOOK_NODE:-${RELEASE_NODE:-livebook_server}}
 if [ -z "${LIVEBOOK_CLUSTER}" ]; then DISTRIBUTION_DEFAULT="sname"; else DISTRIBUTION_DEFAULT="name"; fi