add git

`npm init wasm-app www`実行時のエラー解消のため

refs. rustwasm/create-wasm-app#44 (comment)

Author: dak2

Diff for: Dockerfile

@@ -2,7 +2,7 @@ FROM rust:slim
 
 RUN set -ex \
     && apt-get update \
-    && apt-get install -y curl ca-certificates pkg-config make unzip --no-install-recommends \
+    && apt-get install -y git curl ca-certificates pkg-config make unzip --no-install-recommends \
     && curl -sL https://deb.nodesource.com/setup_16.x | bash - \
     && curl https://rustwasm.github.io/wasm-pack/installer/init.sh -sSf | sh \
     && apt-get update \