tools: toolchain: support dbuild from subdirectory or parent directory of scylla.git
tools: toolchain: support dbuild from subdirectory or parent directory of scylla.git

When building something other than Scylla (like scylla-tools-java or scylla-jmx)
it is convenient to run it from some other directory. To do that, allow running
dbuild from any directory (so we locate tools/toolchain/image relative to the
dbuild script rather than use a fixed path) and mount the current directory
since it's likely the user will want to access files there.
Message-Id: <0ad66ec507eb3ff84c998c49839a240ccbe3ea46@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla