Update Dockerfile

A few changes:

1. apt-get command updated to follow Docker best practices
2. Move the copying of source files to later layers of the Dockerfile since they are more likely to change more often (and thus avoid rerunning of apt-get commands)
3. Add comments to clarify how to modify the file from the student POV

nvmdb/Dockerfile

@@ -3,6 +3,14 @@ FROM gcc:latest
 RUN apt-get update && apt-get install --yes gdb \
     && rm -rf /var/lib/apt/lists/*
 
+# ===== Put things that shouldn't change often here START =====
+
+# e.g.: Installing Rust
+# RUN curl https://sh.rustup.rs -sSf | sh -s -- -y \
+#  && source "$HOME/.cargo/env"
+
+# ===== Put things that shouldn't change often here END =======
+
 COPY . /
 WORKDIR /
 RUN make