Provide warnings/errors during conversions

[jonboulle]

As noted in #20 (comment) , it would be good to be able to warn people about cases when docker2aci has to perform suboptimal translations (for example when converting an exec statement to use /bin/sh)

[jonboulle]

@iaguis since we started down this path with #43 perhaps we can add this more widely now?

[dghubble]

Agreed. I investigated several red herrings before realizing ENTRYPOINT ["./binary"] vs ENTRYPOINT ["/binary"] was the root issue causing problems due to arguments being ignored in a fetched image. A log line would have saved time.
poseidon/matchbox#39

[jonboulle]

Hmm although for your particular issue, since absolute paths are no longer a requirement in the spec, maybe we should relax this conversion now? @iaguis thoughts?

[iaguis]

Makes sense: #114