Add pkg-config to the Docker image

[justinclift]

What type of PR is this?

• Bug Fix

Description

While investigating a different problem with our Docker builds, it turns out the MySQL client install doesn't always work.

Collecting mysqlclient<3.0,>=1.4 (from memsql==3.2.0)
#22 55.46   Downloading mysqlclient-2.2.0.tar.gz (89 kB)
#22 55.46      ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 89.5/89.5 kB 131.2 MB/s eta 0:00:00
#22 55.49   Installing build dependencies: started
#22 57.66   Installing build dependencies: finished with status 'done'
#22 57.67   Getting requirements to build wheel: started
#22 57.87   Getting requirements to build wheel: finished with status 'error'
#22 57.88   error: subprocess-exited-with-error
#22 57.88   
#22 57.88   × Getting requirements to build wheel did not run successfully.
#22 57.88   │ exit code: 1
#22 57.88   ╰─> [24 lines of output]
#22 57.88       /bin/sh: 1: pkg-config: not found
#22 57.88       /bin/sh: 1: pkg-config: not found

Adding the pkg-config Ubuntu package seems to fix the problem.

How is this tested?

• Manually

I've only tested the Docker build process with this, not the full CI tests. They should (in theory) all work unchanged however.

[codecov]

Codecov Report

Merging #6419 (92fb29c) into master (ab71bde) will not change coverage.
The diff coverage is n/a.

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##           master    #6419   +/-   ##
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  Coverage   60.87%   60.87%           
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  Files         155      155           
  Lines       12718    12718           
  Branches     1729     1729           
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  Hits         7742     7742           
  Misses       4747     4747           
  Partials      229      229