chore: pin archlinux package versions (#48)

* Switch from deprecated archlinux/base to archlinux/archlinux
* Pin a specific base image and repository snapshot, as recent
  versions are inoperable under Coder due to a glibc upgrade
  (https://bugs.archlinux.org/task/69563)

images/base/Dockerfile.arch

@@ -1,7 +1,10 @@
-FROM archlinux/base
+FROM archlinux/archlinux:base-20210205.0.15146
 
 SHELL ["/bin/bash", "-c"]
 
+# Workaround broken upstream archive by pinning to older snapshot
+COPY mirrorlist /etc/pacman.d/mirrorlist
+
 # Install baseline packages
 RUN pacman --noconfirm -Syu \
     base \

images/base/mirrorlist

@@ -0,0 +1 @@
+Server=https://archive.archlinux.org/repos/2021/02/01/$repo/os/$arch