docker: update dependency for current build

docker: update dependency for current build

The dependency libssl1.0-dev in the Dockerfile makes docker build fail:

  The following packages have unmet dependencies:
   libmysqlclient-dev : Depends: libssl-dev (>= 1.1.1-1ubuntu2.1~18.04.5~)
  but it is not going to be installed
  E: Unable to correct problems, you have held broken packages.

There seems to be a conflict with different versions of libssl and
libmysqlclient that did not exist previously with Ubuntu 18.04.

Just use the current libssl-dev from Ubuntu 18.04 to fix the build.

Signed-off-by: Pranav Annam <pranavannam@gmail.com>
[rephrased commit message]
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>