title: specify sysctl values using docker-compose
body: <p>I am trying to set a few sysctl values.<br>
  Basically the following </p>
  
  <pre><code>sysctl -w \
     net.ipv4.tcp_keepalive_time=300 \
     net.ipv4.tcp_keepalive_intvl=60 \
     net.ipv4.tcp_keepalive_probes=9
  </code></pre>
  
  <p>in a docker container.<br>
  When log into to the container directly and execute the command, I get the following error</p>
  
  <pre><code>sysctl: cannot stat /proc/sys/net/ipv4/tcp_keepalive_time: No such file or directory
  sysctl: cannot stat /proc/sys/net/ipv4/tcp_keepalive_intvl: No such file or directory
  sysctl: cannot stat /proc/sys/net/ipv4/tcp_keepalive_probes: No such file or directory
  </code></pre>
  
  <p>Then I found out the --sysctl option in <code>docker run</code> in <a href="https://docs.docker.com/engine/reference/commandline/run/" rel="noreferrer">here</a>
  But I did not find the equivalent option via docker-compose. 
  I have few services that start by default so using docker run instead of docker-compose is not an option for me.</p>
  
  <p>Anyone knows of a way to supply --sysctl options to the container via compose?</p>