collectEntries in Groovy works on collections: an optionally passed closure (otherwise identity is used) must return either tuples (two element arrays/lists -- think key and value) or maps and it turns that into a map. E.g.:

[[1,2],[3,4]].collectEntries()
// → [1:2, 3:4]
[[a: 1, b: 2],[c: 3]].collectEntries()
// → [a:1, b:2, c:3]
[1,2,3].collectEntries{ [it, it] }
// → [1:1, 2:2, 3:3]

So this creates maps. But what map exactly? Educated guess: LinkedHashMap, which is also the implementation the Groovy map literal uses. Let's check:

[[1,2],[3,4]].collectEntries().getClass()
// → class java.util.LinkedHashMap

So what if we want another data structure? A not so well know feature (at least to me), is the possibility to pass in an existing map.

So this can be used to pick an appropriate data type:

[3,2,1].collectEntries(new TreeMap()){ [it, it] }
// → [1:1, 2:2, 3:3]

Or to append to an existing map:

[1,2,3].collectEntries([(0): 0]){ [it, it] }
// → [0:0, 1:1, 2:2, 3:3]

My setup to run cargo via Docker as the current user. Put that in your Makefile or as shell alias.

docker run --rm -ti \                            # run, remove afterwards, use terminal
        -w /src -v `pwd`:/src \                  # map cwd to /src and make it the WORKDIR
        --user `id -u`:`id -g` -e USER=`id -u` \ # map to current user
        -e CARGO_HOME=/src/.cargo_registry \     # change the CARGO_HOME into the src
        rust:1.38 \                              # image:version
        cargo

(Note: don't add the comments, the shell won't kike it)