Dockerfile.e2e: fix DOCKER_GITCOMMIT handling

1. There is no need to persist DOCKER_GITCOMMIT,
as it's not needed for runtime, only for build.
So, remove ENV.

2. In case $GITCOMMIT is not defined during build time
(and it happens if .git directory is not present),
we still need to have some value set, so set it to
`undefined`. Otherwise we'll have something like

>  => ERROR [builder 2/3] RUN hack/make.sh build-integration-test-binary
> ------
>  > [builder 2/3] RUN hack/make.sh build-integration-test-binary:
> #32 0.488
> #32 0.505 error: .git directory missing and DOCKER_GITCOMMIT not specified
> #32 0.505   Please either build with the .git directory accessible, or specify the
> #32 0.505   exact (--short) commit hash you are building using DOCKER_GITCOMMIT for
> #32 0.505   future accountability in diagnosing build issues.  Thanks!

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Dockerfile.e2e

@@ -40,12 +40,11 @@ RUN CGO_ENABLED=0 go build -buildmode=pie -o /build/httpserver/httpserver github
 FROM base AS builder
 
 # Set tag and add sources
-ARG DOCKER_GITCOMMIT
-ENV DOCKER_GITCOMMIT=${DOCKER_GITCOMMIT:-undefined}
 COPY . .
 # Copy test sources tests that use assert can print errors
 RUN mkdir -p /build${PWD} && find integration integration-cli -name \*_test.go -exec cp --parents '{}' /build${PWD} \;
 # Build and install test binaries
+ARG DOCKER_GITCOMMIT=undefined
 RUN hack/make.sh build-integration-test-binary
 RUN mkdir -p /build/tests && find . -name test.main -exec cp --parents '{}' /build/tests \;