arm64 images for agentk

Fixes #187 (closed).

Some testing:

$ uname -m
aarch64

# this fails as expected
$ docker run -it --rm registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/agentk:7e3b3db
Unable to find image 'registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/agentk:7e3b3db' locally
7e3b3db: Pulling from gitlab-org/cluster-integration/gitlab-agent/agentk
ab2f6dae3b54: Pull complete
3928c9282920: Pull complete
Digest: sha256:df0e0c8349bf44b757e3b1b4fd81c738fa78eb9ef86ce98c7df1929c610f41b7
Status: Downloaded newer image for registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/agentk:7e3b3db
standard_init_linux.go:211: exec user process caused "exec format error"

# this works fine
$ docker run -it --rm registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/agentk:7e3b3db-arm64
Unable to find image 'registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/agentk:7e3b3db-arm64' locally
7e3b3db-arm64: Pulling from gitlab-org/cluster-integration/gitlab-agent/agentk
c48e4f4f0694: Pull complete
240a42206e99: Pull complete
Digest: sha256:83dfa5f692548f1642a744a6304e401f951c7b2644b0edeb3c03df803ac8c37d
Status: Downloaded newer image for registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/agentk:7e3b3db-arm64
Program aborted: required flag(s) "kas-address", "token-file" not set

# this works fine too
$ docker run -it --rm registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/agentk:7e3b3db-arm64-race
Unable to find image 'registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/agentk:7e3b3db-arm64-race' locally
7e3b3db-arm64-race: Pulling from gitlab-org/cluster-integration/gitlab-agent/agentk
c48e4f4f0694: Already exists
3b35b98e3011: Pull complete
a6164fb406f8: Pull complete
240a42206e99: Pull complete
Digest: sha256:880d3d66001ce204fcff4a4fecf77ae4c03aaa22fa1a3fbb56bd54cd2f4cc18e
Status: Downloaded newer image for registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/agentk:7e3b3db-arm64-race
Program aborted: required flag(s) "kas-address", "token-file" not set