Build zcash on any system w/ Docker+bash via ./contrib/dev-docker/run.sh.

[nathan-at-least]

Please ensure this checklist is followed for any pull requests for this repo. This checklist must be checked by both the PR creator and by anyone who reviews the PR.

• Relevant documentation for this PR has to be completed and reviewed by @mdr0id before the PR can be merged

The documentation lives in ./contrib/dev/docker/README.md.

• A test plan for the PR must be documented in the PR notes and included in the test plan for the next regular release

Test plan A:

1. Make two clones of the same zcashd revision which has this ./contrib/dev-docker feature on a Debian/Ubuntu dev machine which also has docker installed.
2. In the first run ./zcutil/build.sh.
3. In the second, run ./contrib/dev-docker/run.sh
4. Compare the results.

Ideally the results would be byte-wise binary compatible but that's very likely not to be the case. At the very least, both should produce all the same file names, and the full test suite should run on both, and the zcashd --version output of both builds should match.

Test Plan B

Do the same as Test Plan A, except on the first "direct" build, do it on a system without docker, and on the second docker build, do it on a non-debian/non-ubuntu linux system with docker, then compare the results in the same manner.

What this demonstrates: This shows that building with this dev-docker approach allows anyone with docker to build without installing the dev toolchain directly onto their host system.

[nuttycom]

On linux, this will need to be run with sudo (as docker doesn't allow execution without superuser permissions.); however, if you run the whole run.sh script with sudo then id -u returns 0, which then fails the adduser call in the dockerfile.

This should be documented and overrideable.