Fix Spotify not running and building (#555)
Fix Spotify not running and building (#555)

I could neither run the upstream Docker image nor could I build the Dockerfile.

Bug 1
-----

Cannot run the upstream image:

```
docker pull jess/spotify
docker run -it --rm jess/spotify
spotify: error while loading shared libraries: libSM.so.6: cannot open shared object file: No such file or directory
```

Solution to bug 1
-----------------

Install `libsm6`

Bug 2
-----

Cannot build the Dockerfile:

```
cd dockerfiles/spotify
docker build . --pull
[...]
Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).
Executing: /tmp/apt-key-gpghome.k3eQma7IzG/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys 4773BD5E130D1D45
gpg: key 4773BD5E130D1D45: public key "Spotify Public Repository Signing Key <2970eb1fef549b6a4039e7fd2336ba16b18bcbd2@spotify.com>" imported
gpg: Total number processed: 1
gpg:               imported: 1
Hit:1 http://deb.debian.org/debian sid InRelease
Get:2 http://repository.spotify.com stable InRelease [3316 B]
Err:2 http://repository.spotify.com stable InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D1742AD60D811D58
Reading package lists...
W: GPG error: http://repository.spotify.com stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D1742AD60D811D58
E: The repository 'http://repository.spotify.com stable InRelease' is not signed.
The command '/bin/sh -c apt-get update && apt-get install -y 	dirmngr 	gnupg 	--no-install-recommends 	 apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 4773BD5E130D1D45 	 echo "deb http://repository.spotify.com stable non-free" >> /etc/apt/sources.list.d/spotify.list 	    && apt-get update && apt-get install -y 	sa-utils 	  libgl1-mesa-dri 	  libgl1-mesa-glx 	  libpulse0 	  spotify-client 	utils 	    --no-install-recommends 	&& rm -rf /var/lib/apt/lists/*' returned a non-zero code: 100
```

Solution to bug 2
-----------------

Replaced instructions to retrieve the key with the official instructions from https://www.spotify.com/us/download/linux/
jfrazelle/dockerfiles,jessfraz/dockerfiles,jessfraz/dockerfiles,jfrazelle/dockerfiles,jessfraz/dockerfiles,jessfraz/dockerfiles,jessfraz/dockerfiles