We're currently hardcoding various pip dependencies in
`appveyor-install.bat` and `travis-install.sh`.
This commit moves all the dependencies to a `requirements.txt` file in
the root of the project, and makes every install script run `pip install
-r requirements.txt`.
See: https://github.com/resin-io/etcher/pull/1401#discussion_r116547053
Signed-off-by: Juan Cruz Viotti <jviotti@openmailbox.org>
We don't actually use any build context, as we just mount the local
directory as a volume anyway. Omitting it speeds up the container
start-up time. This is an enhanced version of #1173