This PR allows 'minio update' to not only shows update banner
but also allows for in-place upgrades.
Updates are done safely by validating the downloaded
sha256 of the binary.
Fixes#4781
Refer #4759.
Fix this to avoid issues like below
```
docker run --rm minio/minio:edge-armhf version
minio: <ERROR> .... is not compiled by Go >= 1.8.3. ... recompile...
```
We used to release by building directly on the docker
hub auto build process, which is sufficient for edge
but it is not a good idea to do it for stable releases.
Do not build docker release binaries again, but instead
use the released binaries themselves which are signed
and validated.
Benjamin