Add support for OS package manager like apt and apk in Dockerfiles

While your docker support is great to keep the `FROM` directive updated, it could be enhanced by including support for the OS package managers (like APT for Debian and derivatives, APK for Alpine...).

With this addition we could completely rely on dependabot to keep our Docker images updated, instead of having to keep that manually updated.

Example 1 (APT on Ubuntu):

```shell
apt-get install apache2=2.2.20-1ubuntu1 \
apache2.2-common=2.2.20-1ubuntu1 \
apache2.2-bin=2.2.20-1ubuntu1 \
apache2-mpm-worker=2.2.20-1ubuntu1
```

Example 2 (APK on Alpine):

`apk add packagename=1.2.3-suffix`

We could even get fancy and support version constraints, like >=.