Add changelog script and Dockerfile

Dockerfile

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+FROM alpine
+
+RUN apk --no-cache add \
+        git \
+        bash
+
+ADD changelog.sh /bin/
+RUN chmod +x /bin/changelog.sh
+
+ENTRYPOINT /bin/changelog.sh

changelog.sh

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+#!/bin/bash
+
+# Whatever the fuck this was needed
+# MERGE_PREFIX="Merge pull request"
+
+if [ -z "$PLUGIN_OUTPUT" ]; then
+    PLUGIN_OUTPUT="changelog.txt"
+fi
+
+# Set commit range for git log, from previous commit to latest
+GIT_COMMIT_RANGE="$DRONE_PREV_COMMIT_SHA..$DRONE_COMMIT_SHA"
+GIT_COMMIT_LOG="$(git log --format='%s (by %cn)' $GIT_COMMIT_RANGE)"
+
+# Parse log and output generated changelog to output file
+echo "<b>Changelog for build ${MAJOR_MINOR}.${TRAVIS_BUILD_NUMBER}</b>${NEWLINE}" > $PLUGIN_OUTPUT
+
+printf '%s\n' "$GIT_COMMIT_LOG" | while IFS= read -r line
+do
+  echo "- ${line}" >> $PLUGIN_OUTPUT
+done
+
+# Print out Changelog
+cat $PLUGIN_OUTPUT