Dockerfile.arm: Add git to builder.

Dockerfile.arm

@@ -3,7 +3,7 @@
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 FROM rust:alpine AS builder
 
-RUN apk add --no-cache g++
+RUN apk add --no-cache g++ git
 
 WORKDIR /usr/src/libreddit
 
@@ -12,7 +12,8 @@ COPY . .
 # net.git-fetch-with-cli is specified in order to prevent a potential OOM kill
 # in low memory environments. See:
 #     https://users.rust-lang.org/t/cargo-uses-too-much-memory-being-run-in-qemu/76531
-# This is tracked under issue #641.
+# This is tracked under issue #641. This also requires us to install git in the
+# builder.
 RUN cargo install --config net.git-fetch-with-cli=true --path .
 
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