Fix failing Docker builds

Builds are failing due to issues installing systemd (a dependency of gnupg). Here's an example build and the relevant portion of the log:

==> Installing gnupg dependency: systemd
==> Downloading https://github.com/systemd/systemd/archive/v234.tar.gz
==> Downloading from https://codeload.github.com/systemd/systemd/tar.gz/v234
==> Downloading https://github.com/systemd/systemd/commit/227b8a762fea1458547be2cdf0e6e4aac0079730.diff?full_index=1
==> Patching
==> Applying 227b8a762fea1458547be2cdf0e6e4aac0079730.diff
patching file src/core/dbus-execute.c
Hunk #1 succeeded at 17 with fuzz 2 (offset -1 lines).
patching file src/core/manager.h
Hunk #1 succeeded at 19 (offset -1 lines).
Hunk #2 succeeded at 31 (offset -2 lines).
patching file src/core/mount.c
Hunk #1 succeeded at 22 (offset -1 lines).
==> ./autogen.sh
Last 15 lines from /home/dot/.cache/Homebrew/Logs/systemd/01.autogen.sh:
2019-12-28 09:35:04 +0000
./autogen.sh
glibtoolize: error: $pkgauxdir is not a directory: '@@HOMEBREW_CELLAR@@/libtool/2.4.6_2/share/libtool/build-aux'
autoreconf: glibtoolize failed with exit status: 1
READ THIS: https://docs.brew.sh/Troubleshooting
These open issues may also help:
systemd does not build https://github.com/Homebrew/linuxbrew-core/issues/18359
Installing gpg has failed!

Technically this installation issue could probably be worked around but until these dotfiles change substantially that seems like unnecessary effort without much benefit. In the meantime there are several related issues to follow that once resolved should fix this:

nathanshelly / .files

Fix failing Docker builds #38