Specify error behavior for command actions (i.e. COPY, CAPTURE, and RUN)

[vladimirvivien]

Currently, when a COPY, CAPTURE, or RUN action fails, the system decides what to do (in most cases it keeps executing the script). A better approach would be to allow the script author to specify how the command should behave like this:

RUN cmd:'bin/echo "HELLO WORD!"' errexit:"true"

The previous would cause the script to exit if this command fails with an error.