Update alpine Docker tag to v3.14

Dockerfile

@@ -23,7 +23,7 @@ RUN if [ -n "${VIKUNJA_VERSION}" ]; then git checkout "${VIKUNJA_VERSION}"; fi \
 # The actual image
 # Note: I wanted to use the scratch image here, but unfortunatly the go-sqlite bindings require cgo and
 # because of this, the container would not start when I compiled the image without cgo.
-FROM alpine:3.12
+FROM alpine:3.14
 LABEL maintainer="maintainers@vikunja.io"
 
 WORKDIR /app/vikunja/