Docker improvements (#571)

- Copy the current checkout to the Dockerfile.
- Add .dockerignore to exclude files while copying.
- Test tip-of-tree build within CI.
- Run the test suite outside the Docker image.
- Perform extra sanity tests.
- Switch to make commands for CircleCI.
- Improve Docker README.

.dockerignore

@@ -0,0 +1,25 @@
+# Ignore all subdirectories
+*/*
+
+# Allow to run the test script inside the Docker container
+!/docker/test_dockerimage.sh
+
+# Allow the Dockerfile for future re-creation/reference
+!/docker/Dockerfile
+
+# Ignore unnecessary files inside top-level directory
+*.bat
+*.csh
+*.fish
+*.ps1
+*.pyc
+.emscripten
+.emscripten.old
+.emscripten_cache
+.emscripten_cache__last_clear
+.emscripten_sanity
+.emscripten_sanity_wasm
+.flake8
+emscripten-releases-tot.txt
+legacy-*-tags.txt
+README.md