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.
Paul Trunk
Harshavardhana
Nitish Tiwari