Speedup things a bit more

[iyesin]

Here is summary of changes in this PR:

• Option for http proxy configuration. This done mainly for using local caching proxy
• RUN --mount… feature of BuildKit is used to make certain part of builder image cache-able (and tmpfs used to avoid writing man/docs)
• DEB server used in multistrap configs changed to one from https://www.ports.debian.org/mirrors (CDN-backed)
• truncate instead of dd to save some I/O and time
• partitions now have distinct names. Filesystems as well
• bigger /boot to carry more than one kernel version
• MMC/SD fine tuning (please, test!)
• workaround for cheap SanDisk/Samsung MMC's slow initialization (a bit weird but working solution)

[tmolteno]

This looks very good. There are a lot of changes here. Can you confirm that the images are booting on the d1?