`sage -pip install --user ffmpeg` did not make the "`OSError: Error: ffmpeg does not appear to be installed.`" message go away

edit

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 sage -pip install --user ffmpeg did not make the "OSError: Error: ffmpeg does not appear to be installed." message go away, even though it said it had been installed. Here is the full message.

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 <module>
  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.

For reference, here is the code that gives the error:

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()