Allow running this repo as a pre-commit hook

Dockerfile

@@ -0,0 +1,18 @@
+# This file builds ShellCheck for pre-commit.
+#
+# It may also be useful for local development, but it is notably NOT
+# used to build the official ShellCheck docker images.
+
+FROM ubuntu:20.04
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y cabal-install
+RUN cabal update
+
+# Install dependencies separately for more efficient iteration
+COPY ShellCheck.cabal /build/
+RUN cd /build && cabal install --dependencies-only
+
+# Now build the rest
+COPY . /build/
+RUN cd /build && cabal build shellcheck
+RUN find /build -type f -name shellcheck -perm /111 -exec cp -f {} /bin \;
+ENTRYPOINT ["/bin/shellcheck"]