Feature: Silence DL3009,DL3019 for cache mounts

[m-ildefons]

In case the cache directories DL3009 and DL3019 respectively are
provided via BuildKit cache mount, the two rules need not warn a user to
remove the directories.

fixes: #671

How to verify it

The following Dockerfiles should no longer emit warnings DL3009 or DL3019 respectively:

FROM alpine:3.14.0
RUN --mount=type=cache,target=/var/cache/apk apk add -U curl=7.77.0-r1

FROM debian:10
RUN --mount=type=cache,target=/var/lib/apt/lists/ apt-get update && apt-get install foo

[lorenzo]

thanks!