Warning due to no default checked-in toolchain configs for RBE Ubuntu 18.04

I'm trying to use gcr.io/cloud-marketplace/google/rbe-ubuntu18-04 as the base image for a rbe container and I get this warning each time I run bazel:

`
rbe_default not using checked in configs; digest 'sha256:5703843282226b2f7db86afc0b1535f46d411fa72c1d540e1bd0f8064adb0bdb' was picked/selected but no compatible checked in config was found in map '{"sha256:06f8f8e9f97daa1c15466536dc2a7ae6641d16962d7b58a393af8060e460f571": ["10.0.0"], "sha256:09fbb5438d51626dabfe096db381b733af6ed5fd59f07f0a311840598f78019c": ["9.0.0"], "sha256:1ab40405810effefa0b2f45824d6d608634ccddbf06366760c341ef6fbead011": ["10.0.0"], "sha256:1e7bf60f191f6221b010f9338b57936d378baa7b6488dcf5235e2939d62fb9ec": ["10.0.0"], "sha256:29dc13bace3faca2b42e8dbd32c314bed960c313b9e144575eee58338eead9a8": ["10.0.0"], "sha256:2b73cbf679cbf11ed1f782511d3eb8ec7d69049b5947f503c190e9352fd27289": ["9.0.0"], "sha256:2c925275fb30478602cd53651eeaaf015f964ad1b84d3947ed710802f054035b": ["9.0.0"], "sha256:3c104745837918f854415f78f63afe7f680f0876dda837058c38e6eee54e253c": ["10.0.0"], "sha256:3e98e2e1233de1aed4ed7d7e05450a3f75b8c8d6f6bf53f1b390b5131c790f6f": ["9.0.0"], "sha256:4bfd33aa9ce73e28718385b8c01608a79bc6546906f01cf9329311cace1766a1": ["10.0.0"], "sha256:57fbf17cb0d43fb7a00b4e0476750643cb80377e5c38b2e28490d6c69ad8fa2d": ["10.0.0"], "sha256:677c1317f14c6fd5eba2fd8ec645bfdc5119f64b3e5e944e13c89e0525cc8ad1": ["9.0.0"], "sha256:69c9f1652941d64a46f6f7358a44c1718f25caa5cb1ced4a58ccc5281cd183b5": ["9.0.0"], "sha256:6ad1d0883742bfd30eba81e292c135b95067a6706f3587498374a083b7073cb9": ["10.0.0"], "sha256:823aa3cc811b40d8cd7a8df529553ceb8a49bf2adffcebedc4e49dbd8daafca0": ["9.0.0"], "sha256:87d0fa2c56558f2f0d05116e6142b29d9ee509776be5fa9794a57f281b75b14e": ["10.0.0"], "sha256:87e1bb4a47ade8ad4db467a2339bd0081fcf485ec02bcfc3b30309280b38d14b": ["10.0.0"], "sha256:87fe00c5c4d0e64ab3830f743e686716f49569dadb49f1b1b09966c1b36e153c": ["8.0.0"], "sha256:91739a2a3979753111d563fc0202e10e9cd8b9b2ce9552d6a7213892cfe2deb7": ["10.0.0"], "sha256:93f7e127196b9b653d39830c50f8b05d49ef6fd8739a9b5b8ab16e1df5399e50": ["10.0.0"], "sha256:94d7d8552902d228c32c8c148cc13f0effc2b4837757a6e95b73fdc5c5e4b07b": ["9.0.0"], "sha256:98cd34f400a696c0409a3aa0411923b7198aced800a84f23b31f883f8bf407e7": ["9.0.0"], "sha256:9bd8ba020af33edb5f11eff0af2f63b3bcb168cd6566d7b27c6685e717787928": ["8.0.0"], "sha256:9d3104c820537dbf975c78048ddbe71d3f82515cf92b1106ddc552292c187511": ["10.0.0"], "sha256:ac36d37616b044ee77813fc7cd36607a6dc43c65357f3e2ca39f3ad723e426f6": ["10.0.0"], "sha256:aec4629f0856fef325ad03e6b593ccc52eff3328ced6cac351667b85eec48f88": ["9.0.0"], "sha256:bc6a2ad47b24d01a73da315dd288a560037c51a95cc77abb837b26fef1408798": ["9.0.0"], "sha256:d4edc52e8c0171905fc43773846b84d8d6ab4f75354986b82f9eddb6563bbe0f": ["10.0.0"], "sha256:d7bea5c70932edfddafda2da51814a17712585df319bbc11b4d17f662aec6c46": ["9.0.0"], "sha256:da0f21c71abce3bbb92c3a0c44c3737f007a82b60f8bd2930abc55fe64fc2729": ["9.0.0"], "sha256:ec8710e636220c090b84f80a657a61b548dc94d4e3df5e3c42ca048ca74bcfb0": ["10.0.0"], "sha256:f3120a030a19d67626ababdac79cc787e699a1aa924081431285118f87e7b375": ["8.0.0"], "sha256:f5d13baa00009baffb87194cd52cef0165b52d37477093ff72410114664f4380": ["9.0.0"], "sha256:f6fb11fbdc2965f7fef1bcc81565e5bc41a6a91d5ee7a375dbb3a8ea130de5f1": ["9.0.0"], "sha256:fbd499b53a377fe2c6c5e65c33bdecd9393871e19a64eaf785fb6491f31849d3": ["9.0.0"], "sha256:fd5690d000da5759121f28ccbc19ebb4545841d816bbf6a72de482cf3e7ce491": ["10.0.0"]}'
`

What does it involve to add support for 18.04?

Start by tracing the rbe_default handling and the checked-in toolchain configuration map referenced in the warning. Compare the selected Ubuntu 18.04 image digest with the existing entries and determine the required configuration format. Done means the supported 18.04 image is represented and the warning no longer appears for that image.

Written by the indexing model from the issue text.