NOTE: I don't think this PR is ready, since the arm/v6 and arm/v7 images are failing pytest due to missing dependencies (gcc and friends). But it's pretty close.

Closes https://github.com/simonw/datasette/issues/1344 . Using a build-matrix for the platforms and this test, we test all the platforms in parallel. I also threw in container scanning.

Switch pip install to use either tags or commit shas

Notably! This also changes the Dockerfile so that it accepts tags or commit-shas.

```

It's backwards compatible with tags, but also lets you use shas

root@712071df17af:/# pip install git+git://github.com/simonw/datasette.git@0.56
Collecting git+git://github.com/simonw/datasette.git@0.56
Cloning git://github.com/simonw/datasette.git (to revision 0.56) to /tmp/pip-req-build-u6dhm945
Running command git clone -q git://github.com/simonw/datasette.git /tmp/pip-req-build-u6dhm945
Running command git checkout -q af5a7f1c09f6a902bb2a25e8edf39c7034d2e5de
Collecting Jinja2<2.12.0,>=2.10.3
Downloading Jinja2-2.11.3-py2.py3-none-any.whl (125 kB) ```

This lets you build the containers in CI every push for testing, which maybe resolves this problem?

Workflow run example

You can see the results in my workflow here. The commit history is different because I squashed this branch, also in the testing branch I had to change github.com/simonw to github.com/blairdrummond for the CI to pick up my git_sha.

Why did the builds fail?

NOTE: The results of all the tests fail, but for different reasons! A few fail to install Rust, the amd64 passes the tests (phew!) but has critical CVEs which fail the container scan, the Arm/v6 and Arm/v7 seem to fail to install the test dependencies due to missing programs like gcc. (gcc is not sufficient though, as this run indicates)