[docker] fix nproc

Author: simogasp

Dockerfile_deps

@@ -24,7 +24,6 @@ RUN apt-get clean && apt-get update && apt-get install -y --no-install-recommend
         libtool \
         automake \
         libssl-dev \
-        libpng12-dev \
         libjpeg-turbo8-dev \
         libdevil-dev \
         libboost-filesystem-dev \
@@ -41,6 +40,6 @@ RUN wget https://cmake.org/files/v3.17/cmake-${CMAKE_VERSION_FULL}.tar.gz && \
     tar zxvf cmake-${CMAKE_VERSION_FULL}.tar.gz && \
     cd cmake-${CMAKE_VERSION_FULL} && \
     ./bootstrap --prefix=/usr/local  -- -DCMAKE_BUILD_TYPE:STRING=Release -DCMAKE_USE_OPENSSL:BOOL=ON && \
-    make -j8 install && \
+    make -j$(nproc) install && \
     cd /tmp && \
     rm -rf cmake