Add tmp dir for alpine too

[ncsibra]

Hi,

I'm getting errors like these:

time="2020-05-11T05:43:37Z" level=error msg="open /tmp/temp-multibuf-419673259: read-only file system"                                                                                         
time="2020-05-11T05:48:25Z" level=error msg="vulcand/oxy/buffer: error when reading request body, err: open /tmp/temp-multibuf-063517198: read-only file system"                               

Based on this issue: https://github.com/containous/traefik-library-image/issues/45, the problem is that the image does not have a writeable /tmp directory. For scratch, it was fixed here: https://github.com/containous/traefik-library-image/pull/40

Pls fix this in the alpine image too.