I tested this build and it seems like opam init still fails due to missing gcc:

$ sudo docker run -it --rm registry.fedoraproject.org/fedora:27 /bin/bash      
[root@5e2c5f620587 /]# dnf install --enablerepo updates-testing opam && opam init
<snip>
Installing:                                  
 opam                    x86_64          2.0.0-0.8.beta6.fc27             updates-testing          1.9 M
<snip>
[WARNING] Running as root is not recommended                                                             
[NOTE] Will configure from built-in defaults.
Checking for available remotes: none.                                                                   
  - you won't be able to use rsync and local repositories unless you install the rsync command on your
    system.                                                                                             
  - you won't be able to use git repositories unless you install the git command on your system.                                                                                                                  
  - you won't be able to use mercurial repositories unless you install the hg command on your system.   
  - you won't be able to use darcs repositories unless you install the darcs command on your system.    

[WARNING] Recommended dependencies -- most packages rely on these:
            - make                
            - m4                                                                                         
            - cc                                     


=-=- Fetching repository information =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
[default] Initialised                         


=-=- Required setup - please read -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=                    

  In normal operation, opam only alters files within ~/.opam.                                       

  However, to best integrate with your system, some environment variables                           
  should be set. If you allow it to, this initialisation step will update                                
  your sh configuration by adding the following line to ~/.profile:                                 

    test -r /root/.opam/opam-init/init.sh && . /root/.opam/opam-init/init.sh > /dev/null 2> /dev/null || true

  Otherwise, every time you want to access your opam installation, you will
  need to run:                

    eval $(opam env)                                                                                    

  You can always re-run this setup with 'opam init' later.
Do you want opam to modify ~/.profile ? [N/y/f]
(default is 'no', use 'f' to choose a different file)


=-=- Creating initial switch (ocaml-base-compiler) =-=-=-=-=-=-=-=-=-=-=-=-=-=-=

=-=- Gathering sources =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
[ocaml-base-compiler.4.07.0] downloaded from cache at https://opam.ocaml.org/cache

=-=- Processing actions -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
-> installed base-bigarray.base
-> installed base-threads.base
-> installed base-unix.base
[ERROR] The compilation of ocaml-base-compiler failed at
        "/root/.opam/default/.opam-switch/build/ocaml-base-compiler.4.07.0/./configure -prefix
        /root/.opam/default -with-debug-runtime".

#=== ERROR while compiling ocaml-base-compiler.4.07.0 =========================#
# context      2.0.0~beta6 | linux/x86_64 |  | https://opam.ocaml.org#bdce7d8b
# path         ~/.opam/default/.opam-switch/build/ocaml-base-compiler.4.07.0
# command      ~/.opam/default/.opam-switch/build/ocaml-base-compiler.4.07.0/./configure -prefix /root/.opam/default -with-debug-runtime                                                                          
# exit-code    2
# env-file     /tmp/opam-xxx-42/ocaml-base-compiler-42-2c871f.env
# output-file  /tmp/opam-xxx-42/ocaml-base-compiler-42-2c871f.out
### output ###
# Configuring OCaml version 4.07.0
# Configuring for host x86_64-unknown-linux-gnu ...
# Configuring for target x86_64-unknown-linux-gnu ...
# Using compiler cc.
# /root/.opam/default/.opam-switch/build/ocaml-base-compiler.4.07.0/./configure: line 401: cc: command not found                                                                                                  
# Compiler family and version: .
# [ERROR!] Unable to compile the test program.
#  Make sure the C compiler cc is properly installed.



=-=- Error report -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
+- The following actions were aborted
| - install ocaml 4.07.0
+- 
+- The following actions failed
| - build ocaml-base-compiler 4.07.0
+- 
+- The following changes have been performed
| - install base-bigarray base
| - install base-threads  base
| - install base-unix     base
+- 
# Run eval $(opam env) to update the current shell environment
Switch initialisation failed, clean up ? ('n' will leave the switch partially installed) [Y/n] y