Improve build environment's ability to compile software

[shasheene]

Rescuezilla's Dockerfile defines an Ubuntu 18.04 (Bionic) "host environment" where the build takes place. Third-party packages are compiled in this environment and debootstrap is used to create an Ubuntu root filesystem, which is then entered using chroot for further modification.

This architecture is becoming far less suitable now that Rescuezilla currently targets more Ubuntu versions: Ubuntu 20.04 (Focal) x86_64, Ubuntu 20.10 (Groovy) x86_64 and hopefully soon re-introducing Ubuntu 18.04 (Bionic) i386 (the last 32-bit Ubuntu release) before its support window ends in the year 2023.

To be more specific, compiling software targeting all these different releases should not happen in an Ubuntu 18.04 environment because it's linking against different versions than the target environments, which makes building the latest version of software like nbdkit from source much harder.

Side note: In the long-term it would be ideal to have builds of Rescuezilla available for more CPU architectures such as ARM.

The task is to revamp the build environment so software can be correctly compiled from source for the target system. This task is a good time to upgrade the Dockerfile host environment from Bionic to the next Ubuntu long-term support release (Focal). It's also a good time to tackle some related issues:

• task #59 (Setup boot loaders from within chroot, rather than using host environment packages)
• task #72 (Building Rescuezilla ISO image with debootstrap newer than v1.0.106 (2018-07-05)) (doing this in a way that meets Rescuezilla design goals will likely require patches to the debootstrap project)

It's also a good time to compile the latest versions of software like partclone and sfdisk's util-linux from source to better match the package environment on Clonezilla's Ubuntu-based releases. Side note: Rescuezilla already compiles older versions from source for Redo Backup v0.9.8-v1.0.4 compatibility reasons.

This is definitely a big task, but being able to use the latest version of packages mean powerful features can then come to Rescuezilla. For example the latest version of projects like nbdkit will make it easier to expand the supported compression formats of the new Rescuezilla "Image Explorer" feature, which will make the goal of instantaneous image mount of even multi-terabyte images a reality much sooner than waiting for Ubuntu to one day update the dependent packages.