ci: verify pkg-config files actually work (#30)

Change the CI builds to verify the pkg-config files we create actually
work. Some improvements in the `.*ignore` files.

.dockerignore

@@ -0,0 +1,22 @@
+# Common build output directory names
+.build/
+_build/
+build-output/
+cmake-out/
+
+# Common bazel output directories
+bazel-*
+
+# Backup files for Emacs
+*~
+
+# Ignore IDEA / IntelliJ files
+.idea/
+cmake-build-*/
+
+# This is a staging directory used to upload the documents to gihub.io
+github-io-staging/
+
+# Ignore Visual Studio Code files
+.vsbuild/
+.vscode/