fix dockerfile build - upgrade image base from ubuntu 16 to 18

[branciard]

Current dockerfile build failed at apt-get commands with the following error :

Configuration file '/etc/cron.weekly/fstrim'
 ==> Deleted (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ?  Your options are:
    Y or I  : install the package maintainer's version
    N or O  : keep your currently-installed version
      D     : show the differences between the versions
      Z     : start a shell to examine the situation
 The default action is to keep your current version.
*** fstrim (Y/I/N/O/D/Z) [default=N] ? dpkg: error processing package util-linux (--configure):
 end of file on stdin at conffile prompt
Errors were encountered while processing:
 util-linux
E: Sub-process /usr/bin/dpkg returned an error code (1)

Upgrade image base from phusion/baseimage:0.10.2 to phusion/baseimage:0.11 solves it.

Note that this new image base switch from ubuntu 16 to ubuntu 18.

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[branciard]

[clabot:check]

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Many thanks,

Parity Technologies CLA Bot