Lock docker version and require set

[lildude]

I noticed yesterday that our tests have started failing:

  1) Error:
TestClassifier#test_classify_ambiguous_languages:
NameError: uninitialized constant TestClassifier::Set
    /home/runner/work/linguist/linguist/test/test_classifier.rb:53:in `test_classify_ambiguous_languages'

This is caused by a missing require "set" that is needed for the use of Set() in the test. I'm not sure why this is suddenly an issue now without any changes. I can only think we were inadvertently getting this elsewhere.

The docker build is also failing with:

[...]
(6/7) Installing libressl3.3-libtls (3.3.3-r0)
ERROR: libressl3.3-libtls-3.3.3-r0: trying to overwrite usr/lib/libtls.so.20 owned by libretls-3.3.3-r0.
ERROR: libressl3.3-libtls-3.3.3-r0: trying to overwrite usr/lib/libtls.so.20.0.3 owned by libretls-3.3.3-r0.
(7/7) Installing libressl-dev (3.3.3-r0)
Executing busybox-1.33.1-r2.trigger
1 error; 333 MiB in 74 packages
The command '/bin/sh -c apk --update add --virtual build_deps     build-base     libc-dev     linux-headers     cmake     && apk --no-cache add icu-dev libressl-dev     && gem install github-linguist     && apk del build_deps build-base libc-dev linux-headers cmake' returned a non-zero code: 1
Error: Process completed with exit code 1.

This being tracked upstream in https://gitlab.alpinelinux.org/alpine/aports/-/issues/12763 and the workaround is to lock to an earlier version of Alpine, which I'm doing by locking us to 3.13.