Plug causing memory leak - caching

Question

We have an elixir (version 1.8.1) web application running inside docker (Server version 1.12.6, Client version 18.09.2) containers within a rancher cluster (Running within EC2, using the AMI Rancheros 1.4.0). We are using the phoenix framework (Version 1.3).
I implemented a simple html cache using the file system. It's a small plug which checks if the html file for the url requested exists and returns the file if it does. If not, it registers a function to run before the response is sent to save the html response to the cache.
It works very well with response times of 4ms and below in case of a cache hit. BUT it seems the plug did introduce a memory leak. The memory the docker container uses grows over time, depending on the amount of traffic the web app receives. If I have a simple crawler going through the side, the memory grows by about 1MB/Minute.
Interestingly, this does not happen locally on my dev machine but in our staging and production environment.
Here is the full plug:
defmodule PagesWeb.Plugs.Cache do
#moduledoc false
import Plug.Conn
def init(default), do: default
def call(
%Plug.Conn{method: "GET", query_string: query_string, request_path: request_path} = conn,
_default
) do
case page_cached?(request_path, query_string) do
true ->
conn
|> put_resp_header("x-phoenix-cache", "true")
|> put_resp_header("content-type", "text/html; charset=utf-8")
|> send_file(200, "priv/static/html#{uri_to_filepath(request_path, query_string)}")
|> halt()
false ->
conn
|> Plug.Conn.register_before_send(&PagesWeb.Plugs.Cache.save_html_to_cache/1)
|> put_resp_header("x-phoenix-cache", "false")
end
end
def call(conn, _default) do
conn
end
def save_html_to_cache(
%Plug.Conn{request_path: request_path, query_string: query_string, resp_body: resp_body} =
conn
) do
case conn.status do
200 ->
html_file = uri_to_filepath(request_path, query_string)
File.mkdir_p(Path.dirname("priv/static/html#{html_file}"))
File.write("priv/static/html#{html_file}", resp_body)
conn
_ ->
conn
end
end
def read_cached_page(url, query_string) do
case File.open("priv/static/html#{uri_to_filepath(url, query_string)}", [:read, :utf8]) do
{:ok, file} ->
content = IO.read(file, :all)
File.close(file)
content
{:error, _} ->
:err
end
end
def page_cached?(url, query_string) do
File.exists?("priv/static/html#{uri_to_filepath(url, query_string)}", [:raw])
end
defp uri_to_filepath(url, query_string) do
query_string =
case query_string do
"" -> ""
qs -> "-#{qs}"
end
case url do
"/" -> "/index.html"
path -> "#{path}#{query_string}.html"
end
end
end