Docker: update dependencies, introduce Composer

Relates to https://github.com/shaarli/Shaarli/issues/607
Relates to https://github.com/shaarli/Shaarli/pull/612
Relates to https://github.com/shaarli/Shaarli/pull/624

See https://github.com/shaarli/Shaarli/wiki/Server-requirements

Modifications:
- [all][env] set $TERM=dumb to avoid debconf-related issues
- [all][pkg] install ca-certificates
- [all][pkg] cleanup APT cache after installing packages
- [dev] refactor Dockerfile
- [prod][master] refactor Dockerfile
- [prod][master][php] use Composer to resolve PHP dependencies

Signed-off-by: VirtualTam <virtualtam@flibidi.net>