title: `sage -pip install --user ffmpeg` did not make the "`OSError: Error: ffmpeg does not appear to be installed.`" message go away
body: <p>I am running a Docker instance of Cocalc on a Ubuntu 16 server.  Everything seems to be working but I am trying to debug why <code>sage -pip install --user ffmpeg</code> did not make the "<code>OSError: Error: ffmpeg does not appear to be installed.</code>" message go away, even though it said it had been installed.  Here is the full message.</p>
  
  <pre><code>Error in lines 9-9
  Traceback (most recent call last):
    File "/usr/local/sage/local/lib/python2.7/site-packages/smc_sagews/sage_server.py", line 1188, in execute
      flags=compile_flags) in namespace, locals
    File "", line 1, in &lt;module&gt;
    File "/usr/local/sage/local/lib/python2.7/site-packages/smc_sagews/sage_salvus.py", line 2944, in show
      s = show0(objs, combine_all=True)
    File "/usr/local/sage/local/lib/python2.7/site-packages/smc_sagews/sage_salvus.py", line 2903, in show0
      b = show0(a)
    File "/usr/local/sage/local/lib/python2.7/site-packages/smc_sagews/sage_salvus.py", line 2868, in show0
      show_animation(obj, **kwds)
    File "/usr/local/sage/local/lib/python2.7/site-packages/smc_sagews/sage_salvus.py", line 2642, in show_animation
      obj.ffmpeg(t, delay=delay, **kwds)
    File "/usr/local/sage/local/lib/python2.7/site-packages/sage/plot/animate.py", line 885, in ffmpeg
      raise OSError(msg)
  OSError: Error: ffmpeg does not appear to be installed. Saving an animation to
  a movie file in any format other than GIF requires this software, so
  please install it and try again.
  </code></pre>
  
  <p>For reference, here is the code that gives the error: </p>
  
  <pre><code>n = var("n")
  frames = []
  xr = (x, 0, 1)
  for k in srange(1, 50):
      g = plot((sum((-1)^(n-1)*sin(n*x)/n,n,1,k)), xr, color="blue", legend_label='k = %d' % k)
      g += plot(x/2, xr, color="green", legend_label="x/2")
      frames.append(g)
  
  a = animate(frames, ymin=0.0, ymax=1.0, legend_loc=(0.2,0.8))
  a.show()
  </code></pre>