Dockerfile: avoid usage errors

[r00t-]

prevent issues like #619 and #680

[r00t-]

this, together with the comment in the Dockerfile, should hopefully avoid such confusion in the future.

[/vzlogger]$ docker build - <Dockerfile
Sending build context to Docker daemon  3.584kB
Step 1/18 : FROM alpine:latest as builder
 ---> 28f6e2705743
Step 2/18 : COPY . /vzlogger
 ---> 49fdd3c1cd71
Step 3/18 : COPY CMakeLists.txt /vzlogger/
COPY failed: file not found in build context or excluded by .dockerignore: stat CMakeLists.txt: file does not exist

[vzlogger]$ docker build .
(succeeds)

[r00t-]

pulling this up here breaks docker-cache caching of the package installations below :(

maybe just copy CMakeLists.txt to a dummy location, it changes rarely so the impact on the cache should be neglegible.
or accept to fail less early.