Create OTP_VERSION file by `mix release`

Dockerfile

@@ -12,8 +12,6 @@ RUN apk add git gcc g++ musl-dev make &&\
 	mkdir release &&\
 	mix release --path release
 
-RUN echo "${OTP_VERSION}" > release/OTP_VERSION
-
 FROM alpine:3.11
 
 ARG BUILD_DATE

mix.exs

@@ -37,12 +37,21 @@ defmodule Pleroma.Mixfile do
         pleroma: [
           include_executables_for: [:unix],
           applications: [ex_syslogger: :load, syslog: :load],
-          steps: [:assemble, &copy_files/1, &copy_nginx_config/1]
+          steps: [:assemble, &put_files/1, &copy_files/1, &copy_nginx_config/1]
         ]
       ]
     ]
   end
 
+  def put_files(%{path: target_path} = release) do
+    File.write!(
+      Path.join([target_path, "OTP_VERSION"]),
+      Pleroma.OTPVersion.version()
+    )
+
+    release
+  end
+
   def copy_files(%{path: target_path} = release) do
     File.cp_r!("./rel/files", target_path)
     release