⚙️ Add ability to build from Dockerfiles directly

vendor/github.com/asottile/dockerfile/tox.ini

@@ -0,0 +1,18 @@
+[tox]
+envlist = py36,py37,py38,pypy3,pre-commit
+
+[testenv]
+deps = -rrequirements-dev.txt
+commands =
+    coverage erase
+    coverage run -m pytest {posargs:tests}
+    coverage report --show-missing --fail-under 100
+    pre-commit install
+
+[testenv:pre-commit]
+skip_install = true
+deps = pre-commit
+commands = pre-commit run --all-files --show-diff-on-failure
+
+[pep8]
+ignore = E265,E501,W504