Do not patch Dockerfiles in CI

When building the Dockerfiles for development, those images are mainly used to
create a reproducible build-environment. The source code is bind-mounted into
the image at runtime; there is no need to create an image with the actual
source code, and copying the source code into the image would lead to a new
image being created for each code-change (possibly leading up to many "dangling"
images for previous code-changes).

However, when building (and using) the development images in CI, bind-mounting
is not an option, because the daemon is running remotely.

To make this work, the circle-ci script patched the Dockerfiles when CI is run;
adding a `COPY` to the respective Dockerfiles.

Patching Dockerfiles is not really a "best practice" and, even though the source
code does not and up in the image, the source would still be _sent_ to the daemon
for each build (unless BuildKit is used).

This patch updates the makefiles, circle-ci script, and Dockerfiles;

- When building the Dockerfiles locally, pipe the Dockerfile through stdin.
  Doing so, prevents the build-context from being sent to the daemon. This speeds
  up the build, and doesn't fill up the Docker "temp" directory with content that's
  not used
- Now that no content is sent, add the COPY instructions to the Dockerfiles, and
  remove the code in the circle-ci script to "live patch" the Dockerfiles.

Before this patch is applied (with cache):

```
$ time make -f docker.Makefile build_shell_validate_image
docker build -t docker-cli-shell-validate -f ./dockerfiles/Dockerfile.shellcheck .
Sending build context to Docker daemon     41MB
Step 1/2 : FROM    debian:stretch-slim
...
Successfully built 81e14e8ad856
Successfully tagged docker-cli-shell-validate:latest

2.75 real         0.45 user         0.56 sys
```

After this patch is applied (with cache)::

```
$ time make -f docker.Makefile build_shell_validate_image
cat ./dockerfiles/Dockerfile.shellcheck | docker build -t docker-cli-shell-validate -
Sending build context to Docker daemon  2.048kB
Step 1/2 : FROM    debian:stretch-slim
...
Successfully built 81e14e8ad856
Successfully tagged docker-cli-shell-validate:latest

0.33 real         0.07 user         0.08 sys
```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docker.Makefile

@@ -17,24 +17,29 @@ ENVVARS = -e VERSION=$(VERSION) -e GITCOMMIT -e PLATFORM
 # build docker image (dockerfiles/Dockerfile.build)
 .PHONY: build_docker_image
 build_docker_image:
-	docker build ${DOCKER_BUILD_ARGS} -t $(DEV_DOCKER_IMAGE_NAME) -f ./dockerfiles/Dockerfile.dev .
+	# build dockerfile from stdin so that we don't send the build-context; source is bind-mounted in the development environment
+	cat ./dockerfiles/Dockerfile.dev | docker build ${DOCKER_BUILD_ARGS} -t $(DEV_DOCKER_IMAGE_NAME) -
 
 # build docker image having the linting tools (dockerfiles/Dockerfile.lint)
 .PHONY: build_linter_image
 build_linter_image:
-	docker build ${DOCKER_BUILD_ARGS} -t $(LINTER_IMAGE_NAME) -f ./dockerfiles/Dockerfile.lint .
+	# build dockerfile from stdin so that we don't send the build-context; source is bind-mounted in the development environment
+	cat ./dockerfiles/Dockerfile.lint | docker build ${DOCKER_BUILD_ARGS} -t $(LINTER_IMAGE_NAME) -
 
 .PHONY: build_cross_image
 build_cross_image:
-	docker build ${DOCKER_BUILD_ARGS} -t $(CROSS_IMAGE_NAME) -f ./dockerfiles/Dockerfile.cross .
+	# build dockerfile from stdin so that we don't send the build-context; source is bind-mounted in the development environment
+	cat ./dockerfiles/Dockerfile.cross | docker build ${DOCKER_BUILD_ARGS} -t $(CROSS_IMAGE_NAME) -
 
 .PHONY: build_shell_validate_image
 build_shell_validate_image:
-	docker build -t $(VALIDATE_IMAGE_NAME) -f ./dockerfiles/Dockerfile.shellcheck .
+	# build dockerfile from stdin so that we don't send the build-context; source is bind-mounted in the development environment
+	cat ./dockerfiles/Dockerfile.shellcheck | docker build -t $(VALIDATE_IMAGE_NAME) -
 
 .PHONY: build_binary_native_image
 build_binary_native_image:
-	docker build -t $(BINARY_NATIVE_IMAGE_NAME) -f ./dockerfiles/Dockerfile.binary-native .
+	# build dockerfile from stdin so that we don't send the build-context; source is bind-mounted in the development environment
+	cat ./dockerfiles/Dockerfile.binary-native | docker build -t $(BINARY_NATIVE_IMAGE_NAME) -
 
 .PHONY: build_e2e_image
 build_e2e_image: