Add bats

bats/Dockerfile

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+FROM debian:stable-slim
+
+LABEL "name"="bats"
+LABEL "maintainer"="GitHub Actions <support+actions@github.com>"
+LABEL "version"="1.0.0"
+
+LABEL "com.github.actions.name"="Bats for GitHub Actions"
+LABEL "com.github.actions.description"="Bash Automated Testing System for Actions"
+LABEL "com.github.actions.icon"="terminal"
+LABEL "com.github.actions.color"="gray-dark"
+
+COPY LICENSE README.md THIRD_PARTY_NOTICE.md /
+
+RUN apt-get update && \
+  apt-get install --no-install-recommends -y \
+    bats && \
+    apt-get clean -y && \
+  rm -rf /var/lib/apt/lists/*
+
+COPY entrypoint.sh /entrypoint.sh
+
+ENTRYPOINT ["/entrypoint.sh"]

bats/entrypoint.sh

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+#!/bin/sh -l
+
+sh -c "bats $*"