Optimize docker image

* Use 2 stage docker building to produce a tiny python3-slim based docker image with just the compilation results.

.dockerignore

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+.git
+.gitignore
+Dockerfile
+
+#
+#
+#     COPIED FROM .gitignore , please keep it in sync
+#
+#
+
+# Logs
+logs
+*.log
+*.swp
+*.swo
+
+# Runtime data
+pids
+*.pid
+*.seed
+
+# Directory for instrumented libs generated by jscoverage/JSCover
+lib-cov
+
+# Coverage directory used by tools like istanbul
+coverage
+
+# Grunt intermediate storage (http://gruntjs.com/creating-plugins#storing-task-files)
+.grunt
+
+# node-waf configuration
+.lock-wscript
+
+# Compiled binary addons (http://nodejs.org/api/addons.html)
+build/Release
+
+# Dependency directory
+# https://www.npmjs.org/doc/misc/npm-faq.html#should-i-check-my-node_modules-folder-into-git
+node_modules
+
+# Ignore build files
+public
+/errorShots
+/old
+/build