Align qos_enclave build with mono + add reproduction instructions

[r-n-o]

Summary & Motivation (Problem vs. Solution)

Currently our internal mono repo doesn't build qos_enclave in the exact same way than this repo does. Subtle differences lead to mismatch in digests, which means external users who do no have mono access cannot verify remote attestations meaningfully.

Something I didn't expect would have an impact on the final digest:

RUN mkdir -p /qos/src
ADD . /qos/src 
WORKDIR /qos/src/qos_enclave
...build...

yields a different digest than:

ADD . /src
WORKDIR /src/qos_enclave
...build...

And another surprising thing:

ENV USER=user
ENV HOME=/home/user
ENV PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

Adding these also alters the final digests.

Shoutout to @lrvick who put me on the right track and noticed, by using a combo of andrcmdr/aws-nitro-enclaves-image-format-build-extract and diffoscope, that the init binary was the thing causing the EIF file to be different.

How I Tested These Changes

make out/qos_enclave/index.json like...100 times.

[Ulexus]

Crazy that we could have so many little variations like this. Thanks!