Pre-commit.

Add pre-commit hooks. Fix warning from hadolint.

.pre-commit-config.yaml

+# vim:ff=unix ts=2 sw=2 ai expandtab
+---
+repos:
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v2.4.0
+    hooks:
+      - id: check-added-large-files
+      - id: check-merge-conflict
+      - id: trailing-whitespace
+        exclude: \.diff$
+  - repo: https://github.com/adrienverge/yamllint
+    rev: v1.19.0
+    hooks:
+      - id: yamllint
+  - repo: https://github.com/amperser/proselint/
+    rev: 0.10.2
+    hooks:
+      - id: proselint
+        types: [plain-text]
+        exclude: LICENSE
+  - repo: https://www.shore.co.il/git/docker-pre-commit
+    rev: v0.3.0
+    hooks:
+      - id: hadolint

Dockerfile

 FROM alpine:3.10 as downloader
 ARG URL=https://github.com/aptible/supercronic/releases/download/v0.1.9/supercronic-linux-amd64
 ARG SHA1SUM=5ddf8ea26b56d4a7ff6faecdd8966610d5cb9d85
-RUN cd /tmp && \
-    wget $URL && \
+WORKDIR /tmp
+RUN wget $URL && \
     echo "$SHA1SUM  supercronic-linux-amd64" > sha1.sum && \
     sha1sum -c sha1.sum && \
     install -m 755 supercronic-linux-amd64 /usr/local/bin/supersonic && \