update for v0.7.x downloader plugin interface

[mlin]

The upcoming v0.7.x branch of miniwdl has a breaking change to the downloader plugin interface, sorry about this. It just adds two arguments to the entry point to give it access to the effective configuration and the task/workflow logger.

The first commit adds these two arguments and leaves everything else alone.

The second, optional commit gets the s3parcp docker image from the miniwdl configuration loader (which would source it either from the environment variable MINIWDL__S3PARCP__DOCKER_IMAGE or a configuration file [s3parcp] docker_image=...). Also it suggests filling that in at runtime using a WDL input instead of the f-string template. This is more a matter of preference clearly, either way works.

[kislyuk]

LGTM, please merge when ready.

[mlin]

Mostly as a note to self -- here's how I minimally tested this

echo "FROM ubuntu:19.10
RUN apt-get update && apt-get install -y ca-certificates
ADD https://github.com/chanzuckerberg/s3parcp/releases/download/v0.1.2-alpha/s3parcp_0.1.2-alpha_Linux_x86_64.tar.gz /s3parcp/s3parcp.tar.gz
RUN tar zvx -C /s3parcp -f /s3parcp/s3parcp.tar.gz
RUN cp /s3parcp/s3parcp /usr/local/bin" | docker build -t s3parcp -

echo -e 'version 1.0\ntask cat { input { File in } command { cat ~{in} } output { String out=read_string(stdout()) } }' > /tmp/cat.wdl
MINIWDL__S3PARCP__DOCKER_IMAGE=s3parcp:latest miniwdl run /tmp/cat.wdl in=s3://mlin-west/alyssa_ben.txt --dir=/tmp --verbose