PACKAGES=ffmpeg doesn't yield working ffmpeg
I've set PACKAGES=ffmpeg to trigger the install of ffmpeg, but when I drop into the container and exec ffmpeg, I get a linker error:

```
 # ffmpeg
Error relocating /usr/lib/libgnutls.so.30: getrandom: symbol not found
```

In the full output (included below), I notice "ERROR INSTALLING CUSTOM PACKAGES" -- but I don't see any actual errors output?

```
$ sudo docker run -e PACKAGES=ffmpeg -it oznu/homebridge:latest /bin/sh
[s6-init] making user provided files available at /var/run/s6/etc...exited 0.
[s6-init] ensuring user provided files have correct perms...exited 0.
[fix-attrs.d] applying ownership & permissions fixes...
[fix-attrs.d] done.
[cont-init.d] executing container initialization scripts...
[cont-init.d] 10-adduser: executing... 
usermod: no changes

-------------------------------------
GID/UID
-------------------------------------
User uid:    911
User gid:    911
-------------------------------------

[cont-init.d] 10-adduser: exited 0.
[cont-init.d] 20-set-timezone: executing... 
[cont-init.d] 20-set-timezone: exited 0.
[cont-init.d] 30-packages: executing... 
Installing extra packages for alpine linux:  ffmpeg
fetch http://dl-cdn.alpinelinux.org/alpine/edge/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/edge/community/x86_64/APKINDEX.tar.gz
(1/35) Installing sdl2 (2.0.8-r1)
(2/35) Installing libxau (1.0.8-r3)
(3/35) Installing libbsd (0.8.6-r2)
(4/35) Installing libxdmcp (1.1.2-r5)
(5/35) Installing libxcb (1.13-r2)
(6/35) Installing libx11 (1.6.7-r0)
(7/35) Installing alsa-lib (1.1.7-r0)
(8/35) Installing libpng (1.6.35-r0)
(9/35) Installing freetype (2.9.1-r2)
(10/35) Installing fontconfig (2.13.0-r0)
(11/35) Installing fribidi (1.0.5-r0)
(12/35) Installing libass (0.14.0-r0)
(13/35) Installing nettle (3.4-r1)
(14/35) Installing p11-kit (0.23.10-r0)
(15/35) Installing libtasn1 (4.13-r0)
(16/35) Installing gnutls (3.6.4-r0)
(17/35) Installing lame (3.100-r0)
(18/35) Installing opus (1.2.1-r1)
(19/35) Installing libogg (1.3.3-r2)
(20/35) Installing libtheora (1.1.1-r13)
(21/35) Installing libjpeg-turbo (1.5.3-r3)
(22/35) Installing v4l-utils-libs (1.14.2-r0)
(23/35) Installing libxext (1.3.3-r3)
(24/35) Installing libxfixes (5.0.3-r2)
(25/35) Installing libpciaccess (0.13.5-r1)
(26/35) Installing libdrm (2.4.92-r0)
(27/35) Installing libva (2.2.0-r0)
(28/35) Installing libvdpau (1.1.1-r2)
(29/35) Installing libvorbis (1.3.6-r1)
(30/35) Installing libvpx (1.6.1-r1)
(31/35) Installing x264-libs (20180304-r1)
(32/35) Installing x265-libs (2.8-r1)
(33/35) Installing xvidcore (1.3.4-r1)
(34/35) Installing ffmpeg-libs (4.0.2-r0)
(35/35) Installing ffmpeg (4.0.2-r0)
Executing busybox-1.27.2-r7.trigger
OK: 354 MiB in 127 packages

--------------------------------------------------------------
ERROR INSTALLING CUSTOM PACKAGES
--------------------------------------------------------------
Failed to install custom packages. See log above for details.
Please ensure all packages requested are named correctly.

A full list of available packages can be found here:
https://pkgs.alpinelinux.org/packages
--------------------------------------------------------------

[cont-init.d] 30-packages: exited 0.
[cont-init.d] 40-dbus-avahi: executing... 
[cont-init.d] 40-dbus-avahi: exited 0.
[cont-init.d] 45-user-data: executing... 
[cont-init.d] 45-user-data: exited 0.
[cont-init.d] 50-plugins: executing... 
Homebridge: Installing plugins...
npm notice created a lockfile as package-lock.json. You should commit this file.
npm WARN homebridge No description
npm WARN homebridge No repository field.
npm WARN homebridge No license field.

up to date in 0.061s
npm WARN homebridge No description
npm WARN homebridge No repository field.
npm WARN homebridge No license field.

added 1 package in 0.504s


   ╭─────────────────────────────────────╮
   │                                     │
   │   Update available 5.6.0 → 6.4.1    │
   │     Run npm i -g npm to update      │
   │                                     │
   ╰─────────────────────────────────────╯

[cont-init.d] 50-plugins: exited 0.
[cont-init.d] 55-config-ui: executing... 
removed 164 packages in 1.239s
[cont-init.d] 55-config-ui: exited 0.
[cont-init.d] done.
[services.d] starting services
Starting dbus-daemon
[services.d] done.
/bin/sh: can't access tty; job control turned off
/homebridge # Starting Avahi daemon
Found user 'avahi' (UID 86) and group 'avahi' (GID 86).
Successfully dropped root privileges.
avahi-daemon 0.7 starting up.
WARNING: No NSS support for mDNS detected, consider installing nss-mdns!
Loading service file /etc/avahi/services/sftp-ssh.service.
Loading service file /etc/avahi/services/ssh.service.
Joining mDNS multicast group on interface eth0.IPv4 with address 172.17.0.3.
New relevant interface eth0.IPv4 for mDNS.
Network interface enumeration completed.
Registering new address record for 172.17.0.3 on eth0.IPv4.
Server startup complete. Host name is c1a8532df7b9.local. Local service cookie is 1384834663.
-U /homebridge -P /homebridge/node_modules -C
[2018-11-21 05:42:41] Loaded config.json with 0 accessories and 0 platforms.
[2018-11-21 05:42:41] ---
[2018-11-21 05:42:41] Loaded plugin: homebridge-dummy
[2018-11-21 05:42:41] Registering accessory 'homebridge-dummy.DummySwitch'
[2018-11-21 05:42:41] ---
[2018-11-21 05:42:41] Loading 0 platforms...
[2018-11-21 05:42:41] Loading 0 accessories...
Setup Payload:
X-HM://0023ISYWY0GOB
Scan this code with your HomeKit app on your iOS device to pair with Homebridge:
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
                                                      
Or enter this code with your HomeKit app on your iOS device to pair with Homebridge:
                       
    ┌────────────┐     
    │ 031-45-154 │     
    └────────────┘     
                       
[2018-11-21 05:42:41] Homebridge is running on port 51826.
Service "c1a8532df7b9" (/etc/avahi/services/ssh.service) successfully established.
Service "c1a8532df7b9" (/etc/avahi/services/sftp-ssh.service) successfully established.

/homebridge # 
/homebridge # 
/homebridge # ffmpeg
Error relocating /usr/lib/libgnutls.so.30: getrandom: symbol not found
/homebridge # docker CMD exited 127
Got SIGTERM, quitting.
Leaving mDNS multicast group on interface eth0.IPv4 with address 172.17.0.3.
[cont-finish.d] executing container finish scripts...
[cont-finish.d] done.
[s6-finish] syncing disks.
avahi-daemon 0.7 exiting.
[s6-finish] sending all processes the TERM signal.
[s6-finish] sending all processes the KILL signal and exiting.
```