Fix translations and encoding on testserver

[sdennler]

1. Why is this change necessary?

Translations on the testserver (http://docker.team-opencaching.de) work only partialy and when creating a new GeoCache, the attributes are not selectable.

⚠️ The changes in this PR did make it work on my machine - more tests on other machines are needed. ⚠️

2. What does this change do, exactly?

• Install gettext
• Configure the correct user for docker "application" instead of "vagrant"
• Set the default character-set for the connection to MariaDB to "utf8mb4"

3. Describe each step to reproduce the issue or behaviour.

1. Install the testserver (from scratch?)
2. Open http://docker.team-opencaching.de
3. Try to switch the language -> It won't work
4. Try to switch the country -> The list is empty on most pages and incomplete on some.
5. Try to create a new GeoCache and select a Attribute -> it wont work

4. Please link to the relevant issues (if any).

https://opencaching.atlassian.net/browse/RED-1278

5. Checklist

• I have written tests and verified that they fail without my change
• I have squashed any insignificant commits
• This change has comments for package types, values, functions, and non-obvious lines of code

[teiling88]

Looks good and worked on my docker instance, too.

Great work!!

[MacGyver-NRW]

Thanks for fixing @sdennler :)
Work approved and doing well.