Build Docker Images with Bazel targets Built - image

Question

I have a (1) Dockerfile and a (2) C++ project with Bazel
I want to create a docker image that has Bazel targets pre-built within the image, so as to when I power up new containers the Bazel targets are pre-built and I just do Bazel run //hello:hello_world from the container bash.
Dockerfile
# Copy my project with Bazel files to a Docker image, and the
...
RUN bazel --output_user_root=/tmp/hello_project/bazel build //...
...
Within the execution of the Dockerfile, I get the following output which is expected
Loading:
Loading: 0 packages loaded
Analyzing: 2 targets (1 packages loaded, 0 targets configured)
Analyzing: 2 targets (11 packages loaded, 18 targets configured)
INFO: Analyzed 2 targets (15 packages loaded, 60 targets configured).
INFO: Found 2 targets...
[0 / 11] [Prepa] BazelWorkspaceStatusAction stable-status.txt
INFO: Elapsed time: 6.333s, Critical Path: 0.37s
INFO: 11 processes: 6 internal, 5 processwrapper-sandbox.
INFO: Build completed successfully, 11 total actions
INFO: Build completed successfully, 11 total actions
When I run a new container form the Docker Image built previously and then on the container I run
bazel run //hello:hello_world
Instead of using existing pre-built targets it re-builds the targets, which is not required.
Result I expect (not get): Everything is pre-built and just needs to run
INFO: Analyzed target //hello:hello_world (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //hello:hello_world up-to-date:
bazel-bin/hello/hello_world
INFO: Elapsed time: 0.163s, Critical Path: 0.01s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Build completed successfully, 1 total action
Hello World!
Result I get: I re-builds the binaries
[root#4a6bdb57fd79 test-rc]# bazel run //hello:hello_world
Extracting Bazel installation...
Starting local Bazel server and connecting to it...
INFO: Analyzed target //hello:hello_world (15 packages loaded, 60 targets configured).
INFO: Found 1 target...
Target //hello:hello_world up-to-date:
bazel-bin/hello/hello_world
INFO: Elapsed time: 6.255s, Critical Path: 0.38s
INFO: 7 processes: 4 internal, 3 processwrapper-sandbox.
INFO: Build completed successfully, 7 total actions
INFO: Build completed successfully, 7 total actions
Hello World!
How can I make sure, the the bazel run uses same pre-built targets and not build them again before run.

Answer 1

This sounds like non-determinism - in the second execution of Bazel, something is different causing a cache miss.
Some things that can cause it:
different options passed to bazel build vs. bazel test - check your .bazelrc file
actions that include VCS info or a timestamp - make sure you have rules_docker 0.22.0 or later to pick up https://github.com/bazelbuild/rules_docker/commit/2b35b2dd56f0be6cc6b8df957332a31435f6b3ce
One step to diagnose is to use the --explain=log.txt and --verbose_explanations flags to Bazel, then the log file will say why it was rebuilt. However it doesn't have much detail, just something like "a source file has changed".
If you want the power tool for this, there is a way to find out exactly why Bazel didn't get a cache hit - read https://georgi.hristozov.net/til/2020/04/20/compare-bazel-execlogs-to-find-non-deterministic-parts-of-the-build