Fix passing entrypoint as a build arg

Neither ARG or ENV are not expandable in ENTRYPOINT, though since
ENV is available at runtime, we can use it as a work-around here.

Thus, before [0], ENTRYPOINT could be overridden using env by passing
--env BINARY=../../../<path to any binary>.
Now previous default behavior is broken since we don't have ENTRYPOINT
env var. This commit fixes default behavior and allows to modify it
from build arg as was intended with [0].

[0] https://review.opendev.org/c/airship/airshipctl/+/791886

Change-Id: I115df039bc4f845d07f53f68fa3b716d86e8b3a9
Signed-off-by: Andrii Ostapenko <andrii.ostapenko@att.com>

Dockerfile

@@ -37,5 +37,10 @@ ARG BINARY=airshipctl
 ENV BINARY=${BINARY}
 COPY --from=builder /usr/src/airshipctl/bin/${BINARY} /usr/local/bin/${BINARY}
 USER 65534
+# ENTRYPOINT instruction does not expand args from both ENV and ARG.
+# Since variable defined with ENV is available at runtime it will be
+# consumed this way. This also means it may be overridden by passing
+# --env ENTRYPOINT=... to docker run
 ARG ENTRYPOINT=/usr/local/bin/${BINARY}
+ENV ENTRYPOINT=${ENTRYPOINT}
 ENTRYPOINT ${ENTRYPOINT}