How to use the SRTM data from bbbike.org to draw contour lines in private server?

I managed to import the data and configure them to appear on my map.

Steps I followed:

1. This tutorial in order to setup a tile server: https://switch2osm.org/serving-tiles/building-a-tile-server-from-packages/
2. On the "import map data" step, I extracted the part of the map I was interested from http://extract.bbbike.org/ (choose the format OSM -> Protocolbuffer (PBF)) and used this file for my database.
3. At this point you should test that you have a working tile server, that is able to serve tiles if you visit http://ip.of.your.server/osm/0/0/0.png
4. Now it is time to add the SRTM data. Go again to the website http://extract.bbbike.org/ and choose the same part of the map that chose when you extracted the database that you used on step 2. This time, choose the format "SRTM Europe PBF (25m)" if your extract is located in Europe, or "SRTM World PBF (40m)" if your extract is located anywhere else in the world. Download this file.
5. Add the following lines at the end of the files /usr/share/osm2pgsql/default.style and /usr/share/osm2pgsql/osm2pgsql/default.style. These lines will force osm2pgsql in the next step to generate three additional table columns in the database, that are needed for the SRTM data. (probably you don't need to add the lines to both of the files, but I don't know which is the correct file so I added the lines to both of them and it worked)

     # Contour lines
     node,way    contour                 text    linear
     way         contour_ext             text    linear
     way         ele                     text    linear
 

1. Append the SRTM data into the database with the command osm2pgsql --append --slim -d gis srtm-data.pbf

2. Add the following XML text in the file /etc/mapnik-osm-carto-data/osm.xml, after the landcover-line Layer (around line 720 in the file).

   <layer name="srtm-minor" srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over"> <stylename>contours-minor</stylename> <datasource> <parameter name="type"><![CDATA[postgis]]></parameter> <parameter name="table"><![CDATA[(SELECT way,ele,ele as name FROM planet_osm_line WHERE contour_ext = 'elevation_minor') AS "contours-minor"]]></parameter> <parameter name="dbname"><![CDATA[gis]]></parameter> </datasource> </layer> <layer name="srtm-medium" srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over"> <stylename>contours-medium</stylename> <stylename>contours-medium-text</stylename> <datasource> <parameter name="type"><![CDATA[postgis]]></parameter> <parameter name="table"><![CDATA[(SELECT way,ele,ele as name FROM planet_osm_line WHERE contour_ext = 'elevation_medium') AS "contours-medium"]]></parameter> <parameter name="dbname"><![CDATA[gis]]></parameter> </datasource> </layer> <layer name="srtm-major" srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over"> <stylename>contours-major</stylename> <stylename>contours-major-text</stylename> <datasource> <parameter name="type"><![CDATA[postgis]]></parameter> <parameter name="table"><![CDATA[(SELECT way,ele,ele as name FROM planet_osm_line WHERE contour_ext = 'elevation_major') AS "contours-major"]]></parameter> <parameter name="dbname"><![CDATA[gis]]></parameter> </datasource> </layer>

   <style name="contours-minor"> <rule> <maxscaledenominator>70000</maxscaledenominator> <minscaledenominator>2000</minscaledenominator> <linesymbolizer stroke="#9cb197" stroke-width="0.3"/> </rule> </style> <style name="contours-medium"> <rule> <maxscaledenominator>300000</maxscaledenominator> <minscaledenominator>70000</minscaledenominator> <linesymbolizer stroke="#9cb197" stroke-width="0.5"/> </rule> <rule> <maxscaledenominator>70000</maxscaledenominator> <minscaledenominator>2000</minscaledenominator> <linesymbolizer stroke="#747b90" stroke-width="0.5"/> </rule> </style> <style name="contours-major"> <rule> <maxscaledenominator>600000</maxscaledenominator> <minscaledenominator>300000</minscaledenominator> <linesymbolizer stroke="#9cb197" stroke-width="0.7"/> </rule> <rule> <maxscaledenominator>300000</maxscaledenominator> <minscaledenominator>70000</minscaledenominator> <linesymbolizer stroke="#747b90" stroke-width="0.7"/> </rule> <rule> <maxscaledenominator>70000</maxscaledenominator> <minscaledenominator>2000</minscaledenominator> <linesymbolizer stroke="#855d62" stroke-width="0.7"/> </rule> </style> <style name="contours-medium-text"> <rule> <maxscaledenominator>70000</maxscaledenominator> <minscaledenominator>2000</minscaledenominator> <textsymbolizer size="10" fill="#9c9" fontset-name="fontset-1" halo-radius="1" wrap-width="14"><![CDATA[[name]]]></textsymbolizer> <textsymbolizer fontset-name="fontset-0" size="10" fill="#747b90" halo-radius="1" placement="line"><![CDATA[[ele]]]></textsymbolizer> </rule> </style> <style name="contours-major-text"> <rule> <maxscaledenominator>300000</maxscaledenominator> <minscaledenominator>70000</minscaledenominator> <textsymbolizer fontset-name="fontset-1" size="10" fill="#747b90" halo-radius="1" placement="line"><![CDATA[[ele]]]></textsymbolizer> </rule> <rule> <maxscaledenominator>70000</maxscaledenominator> <minscaledenominator>2000</minscaledenominator> <textsymbolizer fontset-name="fontset-1" size="10" fill="#855d62" halo-radius="1" placement="line"><![CDATA[[ele]]]></textsymbolizer> </rule> </style>

3. Restart renderd, and you should have the contour lines shown on the map. Probably you will need to tune the osm.xml file, because I noticed that it gets bloated with the contour lines and the default style provided by osm.xml that already shows too many things on the map.