So I noticed that the TOTAL DATA PULLED line doesn't match the previous log. I re-ran the installation a bunch of times until it was correct, and the error went away:
IMAGE ID [latest]: Pulling from plexinc/pms-docker.
IMAGE ID [b849b56b69e7]: Pulling fs layer. Downloading 100% of 41 MB. Verifying Checksum. Download complete. Extracting. Pull complete.
IMAGE ID [42986ef25bcd]: Pulling fs layer. Downloading 100% of 845 B. Verifying Checksum. Download complete. Extracting. Pull complete.
IMAGE ID [d927c1b717ec]: Pulling fs layer. Downloading 100% of 528 B. Download complete. Extracting. Pull complete.
IMAGE ID [15b86ea20233]: Pulling fs layer. Downloading 100% of 170 B. Verifying Checksum. Download complete. Extracting. Pull complete.
IMAGE ID [f39465de3b1a]: Pulling fs layer. Downloading 100% of 22 MB. Verifying Checksum. Download complete. Extracting. Pull complete.
IMAGE ID [dec716eaf178]: Pulling fs layer. Downloading 100% of 3 KB. Verifying Checksum. Download complete. Extracting. Pull complete.
IMAGE ID [1691bdb34def]: Pulling fs layer. Downloading 100% of 102 MB. Retrying in 5 seconds. Retrying in 4 seconds. Retrying in 3 seconds. Retrying in 2 seconds. Retrying in 1 second. Downloading 100% of 97 MB. Retrying in 10 seconds. Retrying in 9 seconds. Retrying in 8 seconds. Retrying in 7 seconds. Retrying in 6 seconds. Retrying in 5 seconds. Retrying in 4 seconds. Retrying in 3 seconds. Retrying in 2 seconds. Retrying in 1 second. Downloading 100% of 80 MB. Retrying in 15 seconds. Retrying in 14 seconds. Retrying in 13 seconds. Retrying in 12 seconds. Retrying in 11 seconds. Retrying in 10 seconds. Retrying in 9 seconds. Retrying in 8 seconds. Retrying in 7 seconds. Retrying in 6 seconds. Retrying in 5 seconds. Retrying in 4 seconds. Retrying in 3 seconds. Retrying in 2 seconds. Retrying in 1 second. Downloading 100% of 74 MB. Retrying in 20 seconds. Retrying in 19 seconds. Retrying in 18 seconds. Retrying in 17 seconds. Retrying in 16 seconds. Retrying in 15 seconds. Retrying in 14 seconds. Retrying in 13 seconds. Retrying in 12 seconds. Retrying in 11 seconds. Retrying in 10 seconds. Retrying in 9 seconds. Retrying in 8 seconds. Retrying in 7 seconds. Retrying in 6 seconds. Retrying in 5 seconds. Retrying in 4 seconds. Retrying in 3 seconds. Retrying in 2 seconds. Retrying in 1 second. Downloading 100% of 678 KB. Verifying Checksum. Download complete. Extracting.
TOTAL DATA PULLED: 166 MB
But still nothing got installed.
Edit: Actually no, this one was just a glitch. The "invalid tar header" error pops up every time, even when the TOTAL DATA PULLED is correct and there are no "retrying" logs.
I've probably tried it about a hundred times already, but the error is the same.