We can move everything but Liberation fonts build to the container.
Sadly those require newer version of fontforge than the one available in
the Steam RT.
There's some extra logic necessary to assure that font build triggers in
the container with the default/all targets.
This removes the requirement to have afdko and fonttools installed on
the host.
Configure will also test the container engine by trying to run the
selected SDK image.
This may make the first configure a bit slow, as it downloads the image,
but after that the SDK will be cached locally.
Debian-like distributions install AFDKO's executables into libexec and
provide an `afdko` helper to call them.
Python's pip installs the executables in bin.
Let's support both.
* Make it more flexible on the image name,
* Remove the image type support, only Docker is likely to be supported.
* Add target runtime name (scout / soldier), independent of the image.
In steamrt-bootstrap,sh
+ Used quotes to prevent word splitting SC2046
+ Used $() notation rather than legacy backtick SC2006
+ which is non-standard. 'command -v' is builtin SC2230
In configure.sh
+ Assigned to local variable separately to avoid masking return values SC2155
+ Used to quote to prevent glob matching SC2053
+ Used -z command rather than ! -n SC2236
+ Fixed SC2129 which would have a minor performance gain of avoiding constantly opening and closing the makefile.
