Introduce build arguments

[bradbeck]

Allow NEXUS_VERSION and NEXUS_DOWNLOAD_URL to be overridden at build time, e.g.

docker build -t nexus-custom --build-arg NEXUS_VERSION=3.x.y --build-arg NEXUS_DOWNLOAD_URL=http:https://.../nexus-3.x.y-unix.tar.gz .

[joedragons]

+1 tho I didn't run the command (just looked at changes).
This seems super useful for testing snapshots pre-release (for example). Appreciate it.

[DarthHater]

This is cool, I would add a note to the README on usage as well, +1 otherwise

On a separate note we could likely convert a lot of the ENV variables to ARG (I don't know if they really need to be ENV variables).

[jswank]

+0. Having the URL as a parameter is useful (env variables can already be used to control everything else), but I'm not sold on the utility of build args vs. env variables for this.

[bradbeck]

AFAIK, environment variables cannot be specified at build time which is when you would need to have these options available since they are not really changeable at container runtime. Also, the current dockerfile does not expose the download url to be overridden.

[jswank]

In that case, it looks good to me ;)

[jswank]

LGTM