chore(docker): upgrade base image for executor image

[dustinspecker]

This upgrades the executor base image from debian:9.6-slim to
debian:10.3-slim. This fixes a number of CVEs. As of today,
debian:9.6-slim has 10 high, 11 medium, and 60 low CVEs [1], while
debian:10.3-slim has 0 high, 4 medium, and 43 low CVEs [2].

1 - https://snyk.io/test/docker/debian%3A9.6-slim
2 - https://snyk.io/test/docker/debian%3A10.3-slim

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I have written unit and/or e2e tests for my change. PRs without these are unlike to be merged.
• Optional. I've added My organization is added to the USERS.md.
• I've signed the CLA and required builds are green.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #2561 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2561   +/-   ##
=======================================
  Coverage   11.21%   11.21%           
=======================================
  Files          75       75           
  Lines       31838    31838           
=======================================
  Hits         3571     3571           
  Misses      27783    27783           
  Partials      484      484

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c4efb8f...d696520. Read the comment docs.

[alexec]

LGTM

[simster7]

Back-ported to 2.6 and 2.7