Allow setting PREFIX at build time

[ersinakinci]

First of all, thanks so much for putting this together. GDAL is a pain to compile.

I'm using your images with https://github.com/UnitedIncome/serverless-python-requirements to get GDAL working on AWS Lambda. Unfortunately, there's a conflict because both this project's Dockerfile and serverless-python-requirements attempt to use /var/task in the container's file system, the former to install GDAL and the latter to mount a volume. The path is hardcoded in both cases.

Ideally, the mount path should be overridable in serverless-python-requirements. Until then, it wouldn't hurt to make the PREFIX, currently set by a hardcoded ENV in the Dockerfile, overridable at build time by using an ARG. I've created a PR that accomplishes precisely this: #18. The default is still set to /var/task, so it shouldn't break anything.

[vincentsarago]

@earksiinni thanks for the issue.

Sadly this is not a simple as it look. To enable specific libcurl, libjpeg and libpng I had to hardcode the path directly in the shared libray: https://github.com/RemotePixel/amazonlinux-gdal#shared-libraries thus changing this path would break some functionality.

To make it happens I'll need to re-work on the issue to be able to use $origin as done in https://github.com/developmentseed/geolambda/blob/master/Dockerfile#L200

[ersinakinci]

@vincentsarago this makes a lot of sense! I've been in so deep trying to get my toolchain working that I didn't even think about the rpath for deployed binaries. My particular use case doesn't use the GDAL executables (just the shared libraries), but of course this would be a problem for most people.

From the link that you posted, it sounds like you'd be open to having two separate variables, one for the PREFIX and another for the rpath. I modified my Dockerfile, based off of yours, to use RPATH_PREFIX for the latter. Or maybe we should stick with ORIGIN, is that idiomatic?