CI: Remove .dockerignore to ensure reproducible builds

Since go 1.18, built binaries also include VCS information such as the
built commit. This information is also included in the official
binaries. To ensure that the Docker container recreates the same
binaries, the .git folder must also be transferred into the container.
Thus, remove the .dockerignore file.

The copied files must also be owned by the current user within the
container, as git refuses to work otherwise.

.dockerignore

@@ -1,11 +0,0 @@
-# Folders
-.git/
-.github/
-changelog/
-doc/
-docker/
-
-# Files
-.gitignore
-.golangci.yml
-*.md

docker/Dockerfile.release

@@ -5,7 +5,7 @@ FROM --platform=linux/amd64 restic/builder:latest as helper
 ARG TARGETOS
 ARG TARGETARCH
 
-COPY . /restic
+COPY --chown=build . /restic
 RUN go run helpers/build-release-binaries/main.go --platform $TARGETOS/$TARGETARCH --skip-compress
 RUN mv /output/restic_${TARGETOS}_${TARGETARCH} /output/restic