try this, after speaking to Kode

Dockerfile

@@ -11,16 +11,13 @@ RUN \
 	php5-dom \
 	php5-gd \
 	php5-intl \
+	php5-openssl \
 	php5-pdo_sqlite \
 	tar
 
-RUN \ 
- apk add --no-cache --repository http://nl.alpinelinux.org/alpine/edge/testing \
-	mini-sendmail
-	
 # add local files
 COPY root/ /
 
 # ports and volumes
-VOLUME /config /books
 EXPOSE 80 443
+VOLUME /config /books

root/defaults/config_local.php

@@ -1,21 +1,21 @@
 <?php
     if (!isset($config))
         $config = array();
-  
+
     /*
      * The directory containing calibre's metadata.db file, with sub-directories
      * containing all the formats.
      * BEWARE : it has to end with a /
      */
     $config['calibre_directory'] = '/books/';
-	
-	$config['calibre_internal_directory'] = '/books/'; 
-    
+
+    $config['calibre_internal_directory'] = '/books/';
+
     /*
      * Catalog's title
      */
     $config['cops_title_default'] = "COPS";
-    
+
     /*
      * use URL rewriting for downloading of ebook in HTML catalog
      * See README for more information
@@ -30,20 +30,20 @@
      *   X-Accel-Redirect   : For Nginx
      *   X-Sendfile         : For Lightttpd or Apache (with mod_xsendfile)
      *   No value (default) : Let PHP handle the download
-     */ 
-	 
+     */
+
 	 $config['cops_x_accel_redirect'] = "X-Accel-Redirect";
-	
+
     /*
      * use URL rewriting for downloading of ebook in HTML catalog
      * See README for more information
      *  1 : enable
      *  0 : disable
      */
-	
+
 	$config['cops_mail_configuration'] = array( "smtp.host"     => "",
-												"smtp.username" => "",
-												"smtp.password" => "",
-												"smtp.secure"   => "ssl",
-												"address.from"  => "COPS"
-); 
+						    "smtp.username" => "",
+						    "smtp.password" => "",
+						    "smtp.secure"   => "ssl",
+						    "address.from"  => "COPS"
+);