add restart policy to docker compose, add sample dockerfile for demoing features

06-building-container-images/Dockerfile.sample

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+# syntax=docker/dockerfile:1
+# escape=\
+# ^ OPTIONAL "directives" (must be at top if used)
+
+# THIS IS A COMMENT
+
+# ARG is the only instruction that can come before FROM 
+ARG BASE_IMAGE_TAG=19.4
+# ARGs can be overriden at build time 
+# > docker build --build-arg BASE_VERSION=19.3 .
+
+FROM node:${BASE_IMAGE_TAG}
+
+RUN echo "Hey Team 👋 (shell form)"
+RUN ["echo", "Hey Team 👋 (exec form)"]
+
+# --mount allows for mounting additional files
+# into the build context
+# RUN --mount=type=bind ...
+# RUN --mount=type=cache ...
+# RUN --mount=type=secret ...
+# RUN --mount=type=ssh ...
+
+# Available only at build time
+# (Still in image metadata though...)
+ARG BUILD_ARG=foo
+
+# Available at build and run time
+ENV ENV_VAR=bar
+
+# Set the default working directory 
+# Use the convention of your language/framework 
+WORKDIR path/to/the/working/directory
+
+ENTRYPOINT [ "echo", "Hey Team 👋 (entrypoint)" ]
+CMD [ "+ (cmd)" ]