* Inject a proxy into the build providing a snapshot view of the package repo.
When the REPO_SNAPSHOT_STAMP variable is set, the auto/build script will attempt
to launch a transparent HTTP proxy on port 8080, and insert an iptables rule to
redirect all outgoing HTTP requests to this proxy.
The proxy, contained in the `magic-proxy` Python script, examines each request
and silently overrides those pointing to InRelease files or files that are
listed in InRelease files. It will instead provide the contents of the requested
file as it was at REPO_SNAPSHOT_STAMP, by downloading the corresponding asset
"by hash".
Minimize the number of manually installed packages in images by marking
dependencies of metapackages as automatically installed.
Squashed commits:
* minimize-manual.py: Followup with some cleanup, correct permissions
* fixup: Do not assume current directory
* Reorganize minimal-manual file location.
work properly because it doesn't allow for signing key configuration.
Rename it to the more domain-specific EXTRA_PPAS (which is now a
space-separated sequence of <ppa-owner>/<ppa-name> pairs), and fetch
signing keys for those from Launchpad using python3-software-properties.
