Build docker containers

Barebones docker images for kgb and agentk.

rules_docker is used for container building and rules_go is used to build Go binaries. rules_docker uses distroless as minimal base images.

$ make docker-export
bazel run //:gazelle
INFO: Analyzed target //:gazelle (27 packages loaded, 158 targets configured).
INFO: Found 1 target...
Target //:gazelle up-to-date:
  bazel-bin/gazelle-runner.bash
  bazel-bin/gazelle
INFO: Elapsed time: 0.338s, Critical Path: 0.03s
INFO: 0 processes.
INFO: Build completed successfully, 1 total action
INFO: Build completed successfully, 1 total action
bazel run \
                //cmd/agentk:container \
                -- \
                --norun
INFO: Analyzed target //cmd/agentk:container (9 packages loaded, 163 targets configured).
INFO: Found 1 target...
Target //cmd/agentk:container up-to-date:
  bazel-bin/cmd/agentk/container-layer.tar
INFO: Elapsed time: 0.299s, Critical Path: 0.04s
INFO: 0 processes.
INFO: Build completed successfully, 1 total action
INFO: Build completed successfully, 1 total action
Loaded image ID: sha256:af08ee7b694b4a2a8add22b4bca5bc26a8c5467148df37e9e401fd5b6aa74b2b
Tagging af08ee7b694b4a2a8add22b4bca5bc26a8c5467148df37e9e401fd5b6aa74b2b as bazel/cmd/agentk:container
bazel run \
                //cmd/kgb:container \
                -- \
                --norun
INFO: Analyzed target //cmd/kgb:container (36 packages loaded, 201 targets configured).
INFO: Found 1 target...
Target //cmd/kgb:container up-to-date:
  bazel-bin/cmd/kgb/container-layer.tar
INFO: Elapsed time: 0.659s, Critical Path: 0.03s
INFO: 0 processes.
INFO: Build completed successfully, 1 total action
INFO: Build completed successfully, 1 total action
Loaded image ID: sha256:e9d24e910fbee57e2c81e29ad6a053cbde2685ee06b042cb72ff0ff430c78ec9
Tagging e9d24e910fbee57e2c81e29ad6a053cbde2685ee06b042cb72ff0ff430c78ec9 as bazel/cmd/kgb:container
➜  gitlab-agent git:(ash2k/containers) ✗ docker run -it --rm bazel/cmd/agentk:container
ERRO[0000] Program aborted                               error="token file: open : no such file or directory"
➜  gitlab-agent git:(ash2k/containers) ✗ docker run -it --rm bazel/cmd/kgb:container   
ERRO[0000] Program aborted                               error="gRPC.dial Gitaly: invalid connection string: \"\""
➜  gitlab-agent git:(ash2k/containers) ✗