Docker quay usage (brentp#70)

* Update docker usage

README.md

@@ -138,7 +138,7 @@ The simplest way is to [![install with bioconda](https://img.shields.io/badge/in
 It can also be installed with `brew` as `brew install brewsci/bio/mosdepth` or used via docker with quay:
 ```
 docker pull quay.io/biocontainers/mosdepth:0.2.4--he527e40_0
-docker run -v /hostpath/:/opt/mount quay.io/biocontainers/mosdepth:0.2.4--he527e40_0 mosdepth -n --fast-mode -t 4 --by 1000 /opt/mount/sample $bam
+docker run -v /hostpath/:/opt/mount quay.io/biocontainers/mosdepth:0.2.4--he527e40_0 mosdepth -n --fast-mode -t 4 --by 1000 /opt/mount/sample /opt/mount/$bam
 ```
 
 Unless you want to install [nim](https://nim-lang.org), simply download the