build: update alpine packages in Docker image to address CVEs

Author: joshiste

Dockerfile

@@ -49,7 +49,8 @@ RUN echo "$VERSION" > /version.txt && echo "$REVISION" > /revision.txt
 ARG USERNAME=steadybit
 ARG USER_UID=10000
 
-RUN adduser -u $USER_UID -D $USERNAME
+RUN apk update && apk upgrade --no-cache && rm -rf /var/cache/apk/* && \
+    adduser -u $USER_UID -D $USERNAME
 
 USER $USER_UID